Preheat the oven to gas 4, 180C, fan 160°C Grease and line a 20cm square cake tin. Using an electric hand whisk, beat the butter and 175g caster sugar together until light and fluffy. Add the lemon zest, then the eggs, beating well. Fold through the flour, then stir through the raspberry puree. Spoon into the lined tin and bake for 20-25 minutes. Leave to cool in the tin for 5 minutes.
To make the drizzle, mix most of the lemon juice (reserve 2tsp for the icing) and 125g icing sugar. Using a skewer, poke holes through to the bottom of the cake. While still in the tin, pour over the drizzle and leave to cool.
Meanwhile, make candied peel to use as decoration. Put 50g caster sugar in a small pan with 50ml water, bring to the boil, then simmer until the sugar has dissolved. Drop the pared lemon strips into the pan and simmer for 3-4 minutes. Remove from the pan with a fork and wind the longer strips around cocktail sticks to create a spiral effect. Dust with caster sugar and allow to set.
To make the pink icing, mix 75g icing sugar with the reserved 2tsp lemon juice, adding a few drops of water to loosen if necessary. Add enough gel colour for the pink shade of your choice. Remove the cake from the tin and drizzle the pink icing diagonally over the cake for a zig-zag pattern. Decorate with the candied peel. To serve, cut the cake into 18 rectangular bars.