Add flour, sugar, baking powder, and salt to a large bowl. Cut butter into small cubes and add to the bowl.
Break butter into the dry ingredients, pinching between your fingers until butter is fully broken in.
Combine egg, egg yolk, sour cream, buttermilk, and vanilla in a small bowl. Add egg mixture and sprinkles to flour mixture and stir just until a soft dough comes together and no flour streaks remain.
Empty the dough onto a lightly floured work surface and knead together. Optionally, if dough is sticky, chill for 30 minutes to firm it up.
Roll the dough out 1/2-inch thick. Use a floured donut cutter to cut the donuts. Place them on squares of parchment.
Add a quart of oil to a pot and heat to 375 degrees F (190 degrees C).
Meanwhile, for glaze, whisk powdered sugar, milk, and dash of vanilla together until smooth; place glaze in a shallow bowl.
To fry donuts, carefully add 2 to 3 donuts to the hot oil. Fry 2 to 3 minutes per side, until golden brown. Remove to a wire rack and let cool for 2 minutes.
Dip donuts into the glaze while still warm and place on the wire rack to set. Top with more sprinkles and let dry for 5 minutes. Enjoy!
