Preheat the oven to 350 F and grease donut pans well with butter (preferable- for flavor) or nonstick spray. Lightly dust pans with flour. Tap out excess flour.

In a large bowl, sift together the flour, baking powder, nutmeg, sugar and salt.

In a medium bowl whisk together the buttermilk, egg, vanilla, and melted butter. Pour the wet ingredients into the dry ingredients and stir until just combined. Stir in the sprinkles. Be careful not to overmix. Overmixing may cause the sprinkles to bleed. 

Use a piping bag or zip lock bag with a corner snipped off or spoon to fill the dough into the molds evenly. For average sized baked donuts, fill molds half full. For larger donuts, fill them between 2/3 and 3/4 full.

Bake 8-10 minutes until toothpick comes out dry or edges are lightly browned. Check for doneness with a toothpick. Let cool in the pans for 2-3 minutes then remove from pan and let donuts cool completely on wire rack.

In the bowl of a stand mixer fitted with the paddle attachment or in a large bowl using a hand held mixer, beat the room temperature butter on medium until creamy, 1-2 minutes. Turn the mixer to low and add the sifted powdered sugar followed by the heavy cream, vanilla extract, and salt. Turn speed up to high and beat for 2 more minutes. If the frosting is too thick, add a little more heavy cream. If it's too thin, add more powdered sugar. If it's too sweet, more salt can be added to balance out that sweetness. 

Frost donuts or, spoon frosting into a piping bag fitted with a large star tip. Top with more sprinkles!