Preheat the oven to 170°C (Gas Mark 3). Cream the butter and both sugars until light and fluffy. Gradually add the eggs, mixing well. Add the almond extract and lemon zest and mix. Sift the flour, baking powder, ground almonds and salt into the bowl, add the milk and mix until smooth.
Weigh the mixture. Transfer one-third into a greased and lined 20cm square baking tin and spread level. Tint the remaining mixture pale pink using a touch of food colouring paste. Scoop half of this pink mixture (roughly 250g) into a second baking tin of the same size and spread level.
Add another drop of pink food colouring to the remaining batter to make it a deeper shade of pink and spread this into a third baking tin.
Bake the cakes on the middle shelf of the oven for about 8 minutes. You may need to bake these cakes on 2 shelves – one just below the middle of the oven and one above and rotate the tins halfway through cooking. Remove from the oven and allow to rest for 2–3 minutes. Carefully lift out the cakes, put onto wire racks and cool.
Warm the apricot jam in a small saucepan to make it runny, then sieve to remove any lumps. Flip the untinted cake over, peel off the lining paper and spread with a layer of jam. Top with the paler pink cake, remove the paper, then spread jam over the top. Peel the paper off the darker pink cake and lay it, top side uppermost, on top of the paler pink cake.
Cover with a sheet of parchment and a baking sheet, pressing the cakes together, and set aside for a few hours for the flavours to mingle and the slices to meld.
Melt both chocolates in a heatproof bowl. Stir until smooth and allow to cool slightly. Remove the baking sheet and paper from the cake. Spread half of the chocolate over the top of the cake and refrigerate for 10 minutes to set. Lay a clean sheet of parchment over the top of the cake, cover with a baking sheet again and flip the cake over. Spread the remaining chocolate over this untinted side of the cake. Allow to set. To serve, trim the edges and cut into squares