Preheat oven to 350°F (175°C) and line a 12-cup muffin tin with liners
Bring all refrigerated ingredients to room temperature
Make Cheesecake Filling:
Beat cream cheese until smooth (30 seconds)
Add sugar and beat until combined (30 seconds)
Mix in egg yolk and vanilla until just incorporated
Make Muffin Batter:
Whisk together all dry ingredients in large bowl
In separate bowl, whisk melted butter, sugar, eggs, vanilla and buttermilk
Gently fold wet ingredients into dry until just combined (don't overmix)
Assemble Muffins:
Fill each muffin cup halfway with chocolate batter
Add 1 tbsp cheesecake filling to center of each
Cover with remaining batter, filling cups ¾ full
Optional: Sprinkle chocolate chips on top
Baking:
Bake for 18-22 minutes until toothpick comes out clean
Let cool in pan for 5 minutes
Transfer to wire rack to cool completely
Notes
Notes:Store in airtight container at room temperature for 2 days or refrigerate for 5 daysFreeze for up to 3 months - thaw at room temperatureFor best results, use room temperature ingredientsDon't overmix the batter to keep muffins tenderDutch-process cocoa powder recommended for richer flavorCan substitute buttermilk with milk + ½ tbsp lemon juice