Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer). Grease a box mould (approx. 11 x 25 cm; 1 3⁄4 l content). Place a strip of baking paper (approx. 20 x 40 cm) in the baking tin so that the narrow sides remain free. Wash the lemons hot, dab dry and finely grate approx. 3 teaspoons of the zest. Squeeze a total of 100 ml juice.
Mix 250 g softened butter, 250 g sugar, lemon zest and 1 pinch of salt with the whisks of the mixer for about 5 minutes until creamy. Separate 2 eggs, chill the egg white. Stir egg yolk and the remaining 2 eggs one after the other into the sugar-butter-cream. Mix flour, starch and baking powder, alternating with lemon juice.
Cut the dough in half. Stir the grenadine and food colouring into one half of the dough. Pour light-coloured dough into the mould, spread fruit spread in blobs on top. Add the pink dough. Twist both doughs with a fork. Bake the cake in a hot oven for about 1 hour.
Take the cake out of the oven. First cool in the mould for approx. 30 minutes, then carefully lift out and let it cool down. Whip the egg whites until stiff, adding 100 g sugar. Pour the egg whites into a piping bag with a star-shaped spout and squirt them onto the cake as tuffs. Flare with a kitchen gas burner.
TIP for top meringue: The beaten egg white tastes nice and creamy even when sugared and flamed. Therefore, please use only fresh eggs, as the beaten egg white remains raw in the middle.