Separate eggs. Cream butter and 200 g sugar with the whisk of the hand mixer. Stir in lemon zest and egg yolks. Sift flour, starch and baking powder over the mixture and fold in.
Stir in milk and lemon juice. Beat egg whites until stiff and fold in. Grease 4 large, ovenproof cups (approx. 350 ml each). Spread the sponge mixture into the cups and place on the oven rack.
Place in the preheated oven 2nd rail from below (electric cooker: 175 °C/ circulating air: 150 °C/ gas: level 2) Pour hot water into a high casserole dish until the dishes are half full of water.
Bake for 25-30 minutes until golden brown. Caramelise 3-4 tablespoons of sugar in a pan until golden brown, let it cool down a bit. Using a spoon, draw a pattern from the sugar onto a piece of baking paper. Let cool and break into decorative pieces.
Remove the cups, let them cool down briefly and put caramel pieces in. Serve immediately.