Heat the oven to 170C/150C fan/gas 3. To make the brownie tier, butter three 26cm loose-bottomed cake tins and line with baking parchment. Beat the butter, sugar and a pinch of salt for 7 mins until pale and fluffy. Add the eggs, one at a time, beating well after each. Fold in the chocolate and vanilla, then the flour. Spoon into the tins and bake for 40 mins until the outside is slightly flaky with a brownie crust, and the centre is set with no wet crumbs when a skewer is inserted into the middle. Leave to cool completely in the tins. Turn the oven up to 200C/180C fan/gas 6.
To make the middle tier, butter and line two 18cm loose-bottomed cake tins. Beat the butter, sugar and a pinch of salt for 7 mins until light and fluffy. Add the eggs, one at a time, beating well after each. Fold in the flour, baking powder, the zest of 2 lemons and a quarter of the lemon juice. Spoon into the tins and bake for 25-30 mins until lightly golden and firm to the touch. Leave to cool in the tins for 5 mins, then transfer to a wire rack to cool completely.
To make the blueberry tier, butter and line two 10cm round cake tins. Beat the butter, sugar and a pinch of salt for 7 mins until pale and fluffy. Add the eggs, one at a time, beating well after each. Fold in the flour, baking powder, milk and blueberries. Spoon into the tins and bake for 20-25 mins until light golden and firm to the touch. Leave in the tins for 5 mins, then transfer to a wire rack to cool completely.
For the ganache, put the chocolate in a heatproof bowl. Bring the cream to the boil over a medium heat, then pour it over the chocolate, whisking continuously until melted and smooth. Leave to cool slightly.
To make the buttercream, beat the butter, icing sugar and vanilla for 10-12 mins until pale and fluffy. Put 850g buttercream in a bowl and beat in the ganache. Spread a small blob of chocolate buttercream over the middle of the 26cm cake drum and top with a brownie sponge. Spread the top of the sponge with 2-3 tbsp chocolate buttercream, then sandwich with a second brownie sponge. Repeat with 2-3 tbsp chocolate buttercream and the last sponge. Spread the rest of the chocolate buttercream over the top and side in a thick layer using a palette knife. Chill for at least 1 hr.
Stir the rest of the lemon zest and juice into the remaining plain buttercream. Spread a small blob of lemon buttercream over the middle of the 18cm cake drum and top with a lemon sponge. Spread the top of the sponge with 2 tbsp lemon buttercream and the raspberry jam, then top with the second sponge. Spread 400g lemon buttercream over the top and side. Chill for 1 hr.
Spread a small blob of lemon buttercream over the middle of the 10cm cake drum and top with a blueberry sponge. Spread the top of the sponge with the lemon curd and 1 tbsp lemon buttercream, then top with the second sponge. Spread 200g lemon buttercream over the top and side. Chill for 1 hr. Dye 250g of the remaining buttercream deep grey/blue using the blue food colourings. Spread this over the blueberry cake tier. Chill for 1 hr.
Dye any remaining buttercream deep red with the pink and red food colourings. Spread this over the lemon cake tier. Chill for 1 hr.
Remove the brownie tier from the fridge. Warm the apricot jam in a pan to loosen, cool slightly, then brush over the top and side of the cake. Roll the sky blue sugar paste out on a sheet of baking parchment lightly dusted with icing sugar to 40cm (5mm thickness). Use this to cover the brownie tier, smoothing it down the sides. Chill overnight.
For the decorations, start with the snowman. Roll 1 rice crispy square into a large ball. Roll two-thirds of the second square into a medium ball, and what’s left into a small ball. Roll out the white sugar paste. Use this to cover the balls, then roll them in your hands to smooth. Create a hat from the black and green sugar pastes. Use two cocktail sticks and mould brown sugar paste around each to make arms with twig fingers. Leave to set at room temperature for 1 hr. Draw eyes and a mouth on the smallest ball with a black writing pen, and make a small carrot nose from orange sugar paste. Assemble the snowman, push the arms into the sides, and stick on the hat and nose using a little edible glue. Draw buttons down the middle of the snowman using the writing pen.
To stack the cake tiers, first measure about five dowels against the side of the bottom tier, and three or four against the side of the middle tier. Cut the dowels to the same height as the tiers. Insert the dowels in a circle into the middle of the tiers – this will support the tier above. Carefully stack the cake tiers from largest to smallest.
Cut small stars from different coloured sugar pastes. Roll out some white sugar paste and stamp out some clouds. Stick the stars to the bottom tier in a rainbow pattern (use a little edible glue, if needed), then affix the clouds to the side. Roll out the yellow sugar paste and stamp out different sized stars, then stick them to the middle tier. Roll out a little green sugar paste and cut out a tree shape using a small knife, then create a brown tree stand from sugar paste. Decorate the tree with sugar paste off-cuts and edible silver balls, if you like. Roll out a little more sugar paste, cut out two small flags and wrap these around the biscuit sticks. Write messages on the flags using a writing pen, and stick into the middle tier. Top the cake with the snowman, securing it into the top tier using a cocktail stick. Best eaten within a day of decorating. Will keep for up to five days.
BOTTOM TIER (15) 1,332 kcals • fat 69g • saturates 41g • carbs 164g • sugars 141g • fibre 5g • protein 11g •
MIDDLE TIER (10) 856 kcals • fat 46g • saturates 28g • carbs 104g • sugars 88g • fibre 1g • protein 5g •
TOP TIER (with decoration) (6) 953 kcals • fat 42g • saturates 26g • carbs 137g • sugars 113g • fibre 1g • protein 6g • salt 0.5g