Preheat your oven to 220°C (420°F). Lightly grease a donut pan with non-stick spray or butter.
In a medium bowl, sift in the dry ingredients, cake flour, cocoa powder, white and light brown sugar, baking powder, baking soda and salt. Stir together.
In a separate bowl, combine the buttermilk, eggs, vegetable oil and vanilla extract. Add the wet ingredients into the dry and mix until combined. Add in the boiling water and chopped chocolate and fold through until just combined.
Spoon the batter into a piping bag fitted with a large round tip and pipe the batter into each donut mould until ¾ full. Tap the pan to release any air bubbles. Bake for 8-10 minutes until the sponge springs back when pressed. Remove from the oven and leave to cool for five minutes in the donut pan. Invert the pan onto a wire rack to release the chocolate donuts. TIP: If the donuts are sticking, gently run a butter knife around the doughnut's edge to release it. Allow the donuts to cool completely before glazing.
To make the prosecco glaze, sift the powdered icing sugar into a bowl and add in the prosecco. Whisk until combined and smooth. If you want a thicker glaze, then add more powdered icing sugar 30g (¼ cup) at a time. If the glaze is too thick, then add more Prosecco, 15ml (1 tablespoon) at a time until you reach your desired consistency.
Dip the chocolate donuts into the Prosecco glaze or spoon the glaze over the top of each donut. Set onto a wire rack to allow any excess to drip off and sprinkle your choice of sprinkles over the top. Baked Chocolate Donuts are best eaten on the day that they are made.