Prep: Preheat your oven to 350°F (175°C). Grease a 9×5 inch loaf pan and line it with parchment paper for easy removal.
Cream Butter & Sugar: In a large bowl, beat the softened butter and 1 cup of sugar until light and fluffy (about 3 minutes). Add eggs one at a time, followed by the vanilla.
Dry & Wet: Whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the butter mixture, alternating with the sour cream. Mix until just combined.
The Blueberries: Gently fold in the blueberries. (Coating them in a teaspoon of flour prevents them from sinking to the bottom!)
Cream Cheese Filling: In a separate small bowl, beat the cream cheese, sugar, egg yolk, and vanilla until smooth.
Layer & Swirl: Pour half the cake batter into the pan. Drop dollops of the cream cheese mixture on top. Cover with the remaining batter. Use a knife to gently swirl the layers together.
Bake: Bake for 55–65 minutes, or until a toothpick inserted into the cake part comes out clean.
Cool: Let it cool in the pan for 15 minutes before transferring to a wire rack