In a medium bowl, beat the softened cream cheese and sugar until smooth. Add the egg yolk and vanilla extract, mixing until fully combined. Set aside.

In a large bowl, whisk together the flour, sugar, baking powder, baking soda, and salt. In another bowl, combine the melted butter, buttermilk, egg, and vanilla extract. Pour the wet ingredients into the dry ingredients and stir until just combined. Gently fold in the blueberries.

Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners. Spoon half of the muffin batter into each liner, filling them about halfway. Add a spoonful of the cream cheese filling to each muffin cup, then top with the remaining batter.

In a small bowl, mix the flour and sugar for the streusel topping. Cut in the cold butter until the mixture resembles coarse crumbs. Sprinkle the streusel evenly over the muffin batter.

Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean. Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack.