150g butter at room temperature
1 tsp baking powder
a pinch of salt
1 sheet of puff pastry
100g blueberry jam
zest of one lemon
cane sugar & icing sugar for on top of the cake
Baking the cake: 50 minutes at 180C/350F, or until a toothpick inserted in the middle comes out clean.
Other steps: see instruction video.