To begin, preheat your oven to 180°C (fan 160°C) or 350°F. Grease a 20cm (8-inch) tart or pie tin with butter, ensuring the base and sides are evenly coated. This prevents sticking and ensures a clean release once baked.

In a large bowl, cream the softened butter and sugar together until pale and fluffy. Use a wooden spoon or hand mixer for a smooth consistency. This step sets the foundation for a tender crust.

Beat in the egg and vanilla extract until fully incorporated. Don’t worry if the mixture looks slightly curdled, it will come together once the dry ingredients are added.

Sift in the flour, baking powder, and salt. Mix until a soft dough forms, avoid overworking, as it can toughen the crust. Press the dough evenly into the base and up the sides of your prepared tin using floured fingers or the back of a spoon.

Lightly prick the base with a fork to prevent air bubbles. Set aside while you prepare the filling, allowing the dough to rest and firm slightly.

In a separate bowl, whisk together the sour cream, egg, sugar, and vanilla extract until smooth. This custard like mixture will bake into a creamy top layer that contrasts beautifully with the berries.

Evenly scatter the blueberries over the prepared crust. If using frozen berries, there’s no need to thaw, fjust toss them lightly in a tablespoon of flour to prevent excessive bleeding into the filling.

Pour the sour cream mixture gently over the berries, spreading evenly with a spatula to cover the fruit without displacing it. Tap the tin lightly on the counter to release any air pockets.

Bake for 30–35 minutes, or until the edges are golden and the centre is just set. Let cool in the tin for 10 minutes, then serve warm or at room temperature. For an authentic Finnish touch, pair with a spoonful of vanilla sauce or a dollop of whipped cream.