Start by lining the bottom of a 9-inch spring form with baking parchment. Grease the sides with butter. Preheat your oven to 180°C/356°F top-to bottom heat or 160°C /320° circulated heat. Wash the red currants and remove them from the vine. The best way is to use a fork and pull the vines through the prongs of the fork, so the berries fall off.
For the cake base, using a hand or stand mixer, whisk the butter until creamy. Add in the egg, sugar, vanilla extract, lemon juice, and salt, and mix until combined. Now add the flour, baking powder, and milk, and mix to a rather thick dough.
Pour and spread the dough into the prepared form. Then sprinkle the breadcrumbs on top. This will ensure that the bottom does not get soggy with the red currant juice.
Prepare a clean mixing bowl. Ensure there is no grease left in it. You can wipe the bowl with white vinegar to be sure. Whisk the egg whites and salt on high speed until it starts to stiffen. Add the sugar bit by bit. Once the egg white is very stiff, carefully fold in the ground almonds. You do not want to mix too much, as you don't want the egg whites to deflate again.
Divide the egg white mass into two portions. In one portion fold in the red currants and spread it on the cake base.
Use the remaining portion to cover the cake. Place in the preheated oven, on the medium rack.
After 10 minutes you can see the top browning (see pictures). Now cover the cake with a sheet of baking parchment to avoid the cake from getting too brown.
Remove the cake from the oven and leave to cool. If you want you can sprinkle the cake with icing sugar /powdered sugar before serving.