Heat oven to 180C/160C fan/gas 4. Butter and line the bases of two 18-19cm cake tins with baking parchment.
Beat the butter and sugar with an electric whisk until pale and fluffy. Add the eggs, one at a time, and beat well, scraping down the sides of the bowl after each addition. Add the lemon zest, vanilla, flour, milk and a pinch of salt and blend until just combined, then evenly divide the mixture between the tins.
Bake in the centre of the oven for 25-30 mins until a skewer inserted into the middle of each cake comes out clean. Cool the cakes in their tins for 10 mins, then turn out onto a wire rack and leave to cool completely.
To make the buttercream, put the butter in a large bowl and beat with an electric whisk until fluffy. Add the icing sugar 2-3 tbsp at a time until it is all incorporated, adding the vanilla and milk halfway through. Sandwich the cakes together with a little of the buttercream and put on a board. Use the remaining buttercream to cover the sides and top of the cake. Chill for 30 mins.
To decorate, roll out the white fondant icing on a surface lightly dusted with icing sugar until large enough to cover the top and sides of the cake. Drape the fondant icing over the cake and carefully mould to fit. Trim off any excess, wrap and set aside.
Roll out the green icing and cut out a semi-circle, using the base of the cake tin as a guide; this will be the snowman’s hat. Brush one side with a little water and stick on the cake. Cut a strip of green icing and make some markings to make it look like the band of a hat, then trim to the correct length and stick on.
To make the nose, shape a round, flat lump from the orange icing. Mark a smile by gently pressing the rim of a water glass into the fondant. Cut out two circles from the black icing for the eyes, then add a small circle of white icing (from the offcuts) to make the eyes appear to sparkle. To make the scarf, wrap a strip of the blue icing around the base of the cake, then use a knife to add cuts for the fringe. Will keep for up to a week in an airtight container in a cool place.