Preheat oven to 350 degrees.

In a large mixing bowl, combine the dry ingredients, including the flour, brown sugar, granulated sugar, cocoa powder, baking soda, and kosher salt. Mix well.

In a separate mixing bowl, whisk together the wet ingredients, including the buttermilk, eggs, unsalted butter, greek yogurt, apple cider vinegar, and vanilla extract. Mix well.

Pour the wet ingredients into the dry ingredients and mix, but take care not to over-mix - just mix until barely incorporated.

Pour batter into a large gallon-sized Ziplock bag with the very tip cut off (or can use a piping bag, if you have one) and pipe the batter into the doughnut molds. The batter should fill of the 3/4 the mold, as they will rise when baked.

Place in the oven and bake 18-20 minutes, or until a toothpick comes out clean when inserted into the center of a doughnut.

Allow the doughnuts to rest in the mold once removing from the oven for 2-3 minutes, then remove from the mold, and transfer the doughnuts to a cooling rack and allow to cool completely.

Repeat with remaining batter.

Melt the butter in a sauce pan over medium heat, about 2 minutes. Be careful not to let the butter burn, but just to let it melt.

Add the cocoa powder and milk, stirring frequently, until it thickens and comes to a boil, about 2-4 minutes.

Remove the pan from the heat and slowly stir in 2 cups of the confectioners sugar, adding the remaining 1/2 cup, bit by bit, as necessary to make the glaze thick and smooth.

If the glaze gets too thick, you can add a little more milk to loosen.

TIP: I recommend setting up a station which is covered by parchment paper so you can move from the glaze to the cooling rack and add sprinkles without worrying about dripping.

Holding the doughnut with your fingers, dip the tops of each doughnut into the warm glaze, do a little spin by twisting your wrist, and then shake to remove any excess glaze.

Place the glazed doughnuts onto a cooling rack, glaze-side-up, and top with sprinkles (or other toppings if you desire).

Repeat until all the doughnuts have been dipped, flipped, and topped with sprinkles.

Allow doughnuts to set on the cooling rack for for 15-20 minutes to allow the glaze to harden.