Whisk buttermilk, melted butter, eggs, and vanilla in a separate bowl until combined.
Fold wet mixture into dry just until streaks disappear. Gently fold in blueberries.
Spoon batter to fill cups halfway. Add a heaping teaspoon of cream cheese filling.
Cover with remaining batter. Sprinkle tops with coarse sugar, if using.
Bake 18–22 minutes, until golden and a tester shows moist crumbs, not raw filling.
Cool 10 minutes in pan, then transfer to a rack to finish cooling.
Notes
Variation: Add 1/2 teaspoon cinnamon to the batter or swap lemon zest for orange for a warmer note. Storage: Keep covered at room temperature 1 day, or refrigerate up to 4 days; warm briefly before serving.This recipe is an original creation inspired by classic Blueberry Cream Cheese Muffins flavors. All ingredient ratios and instructions are independently developed.