Wash the blackberries and pat them dry, delicately, with a cotton tea towel. Wet and squeeze out a sheet of baking paper so that it adheres well to the cake tin, and line it. Put a layer of blackberries in the bottom of the tin, leaving a gap of 1 cm around the edge, and sprinkle a tablespoon of sugar over the fruit. Blend the remaining blackberries with an immersion blender until they become liquid. In a bowl, beat the eggs and sugar with an electric whisk; when they become smooth and creamy add the grated lemon peel, the flour sifted together with the baking powder, and the butter (melted in a small saucepan over a low heat and then cooled down), continuing to stir with a wooden spoon or a whisk.

Stir in the blackberry juice and, when everything is mixed together thoroughly, pour the mixture into the cake tin on top of the blackberries. Level the surface and bake in a preheated oven, at 180 °C, for roughly 40 minutes (test it by inserting a toothpick – if it comes out clean the cake is cooked). Take it out of the oven and leave it to cool down, then place a plate on top of the cake tin and carefully turn it upside down. Remove the paper, sprinkle with plenty of icing sugar, decorate with the remaining blackberries and, if you like, with a few swirls of whipped cream.