Preheat the oven to 160° C w/ fan.
Line the bottom of the spring form w/ baking parchment & butter the egde.
Melt the butter & set aside to let it cool down.
Grate the lemon & set aside the zest. Squeeze the lemon juice & set aside.
Whisk the eggs & the sugar until foamy.
Add the molten butter & mix again.
Mix the flour, the baking powder, salt & lemon zest.
Combine the egg sugar butter mess & the flour mix - carefully.
Fold in the floured blueberries (cleaned beforehand if necessary)
Pour in the spring form & bake for about 45 min.
Let the cake cool down & peel off the baking parchment.
Add the blueberry jam on top of the cake - however you like.
Stir together the icing sugar & the lemon juice & add on top of the cake - however you like.