In a medium bowl, stir together the flour, sugar, baking powder, and salt. In a small bowl, stir together the buttermilk, vanilla extract, melted butter, sour cream, and the eggs. Pour the wet ingredients into the dry.

Stir together for just a couple of turns. While there are still pockets of dry flour in the mixture, toss in your blueberries. Continue to stir together until no dry pockets remain, but do not over-mix.

Cover your bowl and let the batter rest on your counter for 30 minutes.

In a small bowl, whisk together the cream cheese, sugar, vanilla extract, and egg yolk until you have a smooth mixture.

Preheat your oven to 425 degrees. Spray your baking pan with baking spray. Line with paper cups.

Once your batter has rested for a full 30 minutes, scoop about half of the batter evenly into your prepared pan, add a dollop of the cheesecake filling, topped with a dollop of the blueberry jam to each cup. Top each muffin tin cup with the remaining batter, spreading evenly.

The muffin cups should be very full here. Generously sprinkle the tops with the coarse sugar.

Bake for 22-26 minutes, or until the muffins are puffed, lightly golden brown. Cool the muffins completely in the pan.