Brown the butter: take your 4 tbsp of butter, cut it into slices, and place them into a skillet over medium heat. Melt the butter, stirring frequently. It will begin to sputter and crackle. Continue stirring continuously to prevent burning. After a while, the crackling will subside and butter will begin to foam. Keep stirring. Once you see brown flecks and the melted butter turns a rich amber color, immediately remove it from heat and pour the butter into a separate, heatproof bowl. Let the butter cool down before using.
Preheat the oven to 350°F and grease your donut pan(s).
In a large bowl, whisk together the flour, baking powder, baking soda, and salt until combined. These are your dry ingredients.
Heat the buttermilk in a microwaveable cup or bowl for 10-15 seconds to bring it to room temperature.
In a separate bowl, whisk together the sugar, browned butter, eggs, vanilla, and almond extract together until creamy and smooth. Then slowly whisk in the buttermilk until it is fully combined.
Create a well in your bowl of dry ingredients, and then pour the wet ingredients into it. Use a whisk to bring the batter together until the flour is just combined. Do not over-mix--some lumps here and there are okay. Fold in the rainbow sprinkles.
Transfer the batter into a piping bag or use a large zip-loc bag with a corner snipped off.
Pipe the batter into the donut pan, filling each to just over half full. Bake for 10-11 minutes, or until the donuts are set.
Let the donuts cool for 5-10 minutes in the pan, before flipping them out onto wire racks.
To make the glaze, whisk the powdered sugar, milk, and vanilla together until smooth. Add as much milk as needed to reach your desired consistency.
Dip the top side of each donut into the glaze, then garnish with more sprinkles. Enjoy!