In a large bowl, mix together the butter, grated almond paste, and sugar until smooth and creamy.
An electric mixer works best here, but don’t overbeat, it just needs to come together into a uniform mixture.
Add the eggs one at a time, mixing well between each.
In a separate bowl, combine the flour, baking powder, and vanilla sugar (or extract). Add to the wet ingredients and stir until just combined.
Pour the batter into the prepared tin and smooth the top.
Scatter blueberries evenly over the batter, then sprinkle flaked almonds generously on top.
Bake for 25–30 minutes, or until a toothpick inserted in the center comes out clean. The cake will still be a little soft around the berries, this is normal.
Let cool slightly before serving. Delicious warm or at room temperature.