Heat oven to 180C/160C fan/gas 4 and grease 3 x 20cm sandwich tins, lining the bases with baking parchment and greasing the parchment too.
To make the sponge, put the butter and sugars in a large bowl and beat with an electric whisk until light and fluffy, about 5 mins. Add the eggs, a little at a time, beating until fully incorporated before adding more. In another bowl, mix the flour, cornflour, spices and baking powder. Stir in the dry ingredients in 3 additions, alternating with the milk. Divide the batter among the prepared tins, smooth the surfaces and bake for 25-30 mins or until a skewer inserted into the middle of the cakes comes out clean. Allow the cakes to cool in the tins for 10 mins, then turn out onto a wire rack, peeling off the parchment, to cool completely.
For the caramelised pears, peel, core and dice them into small cubes, tossing with the lemon juice as you go. Tip into a frying pan with the maple syrup and butter, and cook over a medium-high heat until the pears have softened and most of the liquid has evaporated. Set aside to cool.
For the buttercream, put the egg whites and sugar in a large heatproof bowl (the bowl of your tabletop mixer, if you have one) and place over a pan of gently simmering water. Whisk until the sugar has fully dissolved - you can test this by dipping two fingers into the bowl and rubbing them together; if you can’t feel any grains of sugar, the mixture is ready. Remove the bowl from the pan and beat with an electric whisk (or use your tabletop mixer) until a thick meringue has formed. Continue whisking until the meringue has cooled to room temperature.
Slowly beat in the butter, 1 tbsp at a time. The mixture should now look like a silky- smooth buttercream – if it doesn’t, continue to whisk until it does. If it still refuses to thicken, it may be the mixture is still too warm, so chill for 10 mins, then continue whisking. Whisk in the maple syrup last.
To assemble the cake, place the first sponge on a cake board or serving plate and spread with a thin layer of the buttercream. Top with half of the caramelised pear chunks, discarding any leftover liquid. Repeat, finishing with the third sponge.
To finish, spread a very thin layer of the buttercream over the sides and top of the cake – this is called ‘crumb-coating’ (see tip). Now spread a thick layer of buttercream on top of the cake. Put the remaining buttercream in a piping bag fitted with a small petal piping tip (see tip). Hold the piping bag with the wide end of the nozzle touching the side of the cake, at the very bottom. Gently wiggle the bag side to side, back and forth, in a zigzag motion, working up the cake in a strip. Repeat, starting next to the previous strip each time, working all the way around the cake until it’s entirely covered. To finish, pipe 1 row of buttercream around the edge of the top of the cake (which will neaten any rough edges at the top of your zigzags). If you like a little sparkle, finish the cake with a coat of edible gold spray. Best served within 2 days of baking, but the cake will keep for up to 4 days.