
Caramel Topping:
1 tablespoon unsalted butter, plus more for the pan
2/3 cup sugar
3 cups blueberries
Cake:
1 3/4 cups cake flour
1 teaspoon baking powder
1/2 teaspoon bicarbonate of soda
1/2 teaspoon fine salt
115g (8 tablespoons) unsalted butter, at room temperature
1 cup sugar
2 large eggs
1 teaspoon pure vanilla extract
1 teaspoon lemon zest
3/4 cup milk
1/4 cup sour cream