Preheat oven to 350 degrees. Spray a donut pan with non-stick oil and set aside. If using a zip-top bag for piping, snip one corner away with a pair of scissors.
Add flour, funfetti sprinkles, baking powder, and sea salt to a large bowl and whisk to combine (about one minute).
Add eggs, sugar, milk, melted butter, and vanilla extract to another bowl and mix until uniform (about four minutes).
Add wet mixture to dry flour mixture and stir by hand until just combined and no dry pockets remain.
Transfer cake donut batter into a piping bag or prepared zip-top bag. Pipe batter into prepared donut pan so that tops are about even with the edges of each cavity in the pan.
Place pan on center rack of oven and bake for about 20 minutes. Tops of donuts will be firm and dry to the touch but will not brown. When fully baked, a toothpick inserted into the center of a donut should come out dry.
For the Glaze
While donuts bake, add milk, sifted powdered sugar, and vanilla extract to a bowl and mix until uniform and smooth.
Allow donuts to cool for 5-10 minutes then dip each one (while still warm) into the glaze. The glazed donuts will look glossy. Place onto a cooling rack to allow the donuts to cool and glaze to harden.
Dip each cooled donut into glaze again then place back on cooling rack and sprinkle funfetti sprinkles over top to decorate.
Notes
Grab a mixing bowl, whisk, and silicone spatula to mix up the funfetti birthday cake batter. Alternatively, you can use a stand mixer. Just be sure not to over mix! You'll also need a cake donut pan and wire cooling rack. Use a piping bag to transfer the batter to the pan or a zip-top bag with one corner snipped off. I use the same tools to make all kinds of baked donuts, including mimosa flavored ones.What's the difference between cake donuts and regular donuts? Cake donuts are baked and have a more dense, cakey texture. Regular donuts are fried pastries that are made with yeast or puff pastry and thus, are more airy and light.