01 - Preheat oven to 350°F. Grease a 9-inch round cake pan with butter and line the bottom with parchment paper to prevent sticking.
02 - Pour melted butter into the bottom of the prepared pan. Sprinkle sugar evenly over the butter. Arrange blueberries in a single layer over the sugar and drizzle with lemon juice. Set aside.
03 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt until thoroughly blended. Set aside.
04 - In a large bowl, beat softened butter and granulated sugar together using an electric mixer or whisk until pale and fluffy, approximately 3-4 minutes.
05 - Add eggs one at a time, beating well after each addition until fully incorporated. Mix in vanilla extract.
06 - Gradually mix in half of the dry ingredients. Add sour cream and milk, then stir in the remaining dry ingredients until just combined. Avoid overmixing to prevent a tough texture.
07 - Carefully spoon the batter over the blueberry layer in the pan. Gently spread the batter to create an even surface, being careful not to disturb the fruit layer beneath.
08 - Bake for 35-40 minutes until the cake is golden brown and a toothpick inserted into the center comes out clean or with just a few moist crumbs.
09 - Let the cake cool in the pan for 10 minutes. Run a sharp knife around the edge to loosen. Place a serving plate over the pan and quickly invert to release the cake. Peel off the parchment paper.
10 - Allow the cake to cool for at least 20 minutes before slicing. Serve warm or at room temperature.