If necessary, thaw the blueberries on kitchen paper, which will absorb the moisture. Pat them dry with more kitchen paper when ready to use. Make sure that the butter is soft, you should take it out of the fridge at least an hour before starting to prepare the cake batter.

Preheat the oven to 180 degrees Celsius/ 350 degrees Fahrenheit. Thoroughly butter a casserole dish of approximately 25x20 cm/ 10x8 inches.

Beat the soft butter and the sugar until light and fluffy, 3-4 minutes. Add the eggs and mix well again. Add sour cream, lemon juice, and zest, and mix shortly again. Mix the flour and the baking powder together, add them to the butter mixture and incorporate shortly. Mix together the blueberries and 1 tablespoon sugar. Add them to the batter and incorporate them carefully.

Pour the batter into the prepared casserole dish and bake for 25 to 35 minutes. Check after 25 minutes already, when the cake is baked through the toothpick inserted into the middle of the cake should come out clean. The top of the blueberry sour cream cake should be golden brown.

Leave to cool slightly and cut the squares directly in the dish before serving. Serve lukewarm or at room temperature. The blueberry sour cream cake is best served on the day you baked it.