Preheat the oven to 325 degrees. Coat the donut baking tray with cooking spray.
Make the donuts by mixing the flour, sugar, baking powder, and salt in a bowl.
Add the buttermilk, egg, vanilla, and melted butter. Mix until well combined.
Transfer the batter into a large zip lock bag. Snip off a corner of the bag and pipe the batter evenly into a 6-cup donut pan.
Bake the donuts in the oven for 9-10 minutes or until the dough is set and springy (it won't be golden brown).
Let them sit for 1-2 minutes, then place the donuts on a baking rack to cool a bit.
Meanwhile, make the chocolate glaze in a heatproof bowl over a small pot of simmering water, and melt the butter and chocolate chips (if using a bar, chop them into small pieces).
Remove the bowl from the heat and whisk in the powdered sugar, vanilla, and hot water until smooth.
Glaze the donuts by dipping each one face-down into the glaze.
Immediately garnish with sprinkles, then let them cool on a wire rack, chocolate side up. Enjoy!!