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. This will help everything come together before baking, and it also helps to ensure you get high-domed muffins.

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- you want the spray to coat the top of the pan mostly to prevent overhang from sticking. 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, and spread the batter to completely cover the bottom of the muffin tin cups. Add a dollop of the cheesecake filling, topped with a dollop of the blueberry jam to each cup. Then top each muffin tin cup with the remaining batter, spreading evenly.

The muffin cups should be very full here, with the batter coming up almost the top of the cups. Generously sprinkle the tops with the coarse sugar.

Bake for 22-26 minutes, or until the muffins are puffed, lightly golden brown, and a toothpick comes out with just a few crumbs on it. Once removed from the oven, use a butter knife to ensure that any overhanging parts don't stick to the pan. Cool the muffins completely in the pan, and enjoy!