Lightly butter a 12-cavity donut pan, or 2 6-cavity donut pans. Preheat oven to 350ºF.

In a large mixing bowl, whisk together the flour, cocoa, baking powder, and salt. In a separate bowl, whisk together the sugar, buttermilk, melted butter, eggs, and vanilla extract. Add the wet ingredients to the dry ingredients and fold them together with a rubber spatula, mixing just until combined.

Fill a large ziplock bag with the donut batter and cut a hold in one corner of the bag. Squeeze the donut batter into the prepared pans, filling each cavity about 2/3 full of batter. Bake in the preheated oven for about 15 minutes, or until a toothpick inserted in the center of a donut comes out clean. Set donuts aside to cool in the pans for 10 minutes.

While the donuts cool, add the semisweet chocolate, butter, corn syrup and water to a small saucepan. Heat over medium-low heat, stirring constantly, until the chocolate and butter melt and a smooth glaze forms. Remove glaze from the heat.

Once the donuts have cooled, remove them from the pans and set them on a cooling rack. Using a small spatula or spreader, spread about 2 Tablespoons of the chocolate glaze on each donut, allowing some of the glaze to drip down the sides of the donuts. Add sprinkles to the donuts right away, before the glaze sets, if desired.