1. Preheat the oven to 350°F. Grease a 6-cavity doughnut pan with non-stick cooking spray.
2. In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt.
3. In another medium bowl, whisk the melted butter, egg, sugars, vanilla extract, and buttermilk.
4. Gradually add the dry ingredients to the wet ingredients, mixing until the batter is smooth. Fold in the finely chopped dark chocolate.
5. Spoon a small amount of batter into each doughnut cavity to cover the bottom.
6. In a microwave-safe bowl, combine the dark chocolate chips and heavy cream. Microwave in 20-second intervals, stirring until smooth and melted.
7. Add a small spoonful of the chocolate filling to each doughnut cavity, then cover with more batter.
8. Bake for 10-12 minutes, until a toothpick inserted into the outer edge comes out clean.
9. Let the doughnuts cool in the pan for 5 minutes before transferring to a wire rack to cool completely.
10. For the glaze, whisk together icing sugar and cocoa powder, blending with 2 tablespoons of cream. Add corn syrup and gradually incorporate the rest of the cream until smooth.
11. Once the doughnuts have cooled, dip the tops into the glaze and decorate with sprinkles if desired.