Step 1: Prepare the Cream Cheese Filling. In a mixing bowl, combine cream cheese, one egg yolk, and powdered sugar. Blend these ingredients together until perfectly smooth and creamy, ensuring there are no lumps. Once well-mixed, scoop dollops of the filling onto a parchment-lined plate and place them in the freezer while you prepare the muffin batter.

Step 2: Make the Crumb Topping. In a separate bowl, cut cold butter into small pieces, then mix in flour and sugar using your fingers or a pastry cutter until the mixture resembles coarse crumbs. Set this mixture aside.

Step 3: Mix the Wet Ingredients. In a stand mixer, beat together butter and oil with brown sugar on medium speed until light and fluffy, about 2-3 minutes. Gradually add in the egg whites one at a time, ensuring each is fully incorporated before the next addition.

Step 4: Combine the Dry Ingredients. In a separate bowl, whisk together the cake flour, cocoa powder, baking powder, baking soda, and salt. Gradually add this dry mixture to the wet ingredients in the mixer, alternating with buttermilk.

Step 5: Fold in the Heavy Cream. In another bowl, whip the heavy cream until soft peaks form. Carefully fold the whipped cream into the muffin batter using a spatula until well combined.

Step 6: Fill the Muffin Pans. Preheat your oven to 425°F. Grease a muffin tin thoroughly. Pour the muffin batter into each cup, filling them halfway, then push the frozen cream cheese dollops into the center, covering them with the remaining batter and sprinkle crumb topping on top.

Step 7: Bake the Muffins. Bake at 425°F for 6 minutes, then reduce to 350°F and bake for another 18 to 22 minutes until a toothpick comes out clean.

Step 8: Cool and Serve. Allow muffins to cool in the tin for about 10 minutes before transferring to a wire rack to cool completely.