01 - In a large mixing bowl, blend together the flour, sugar, yeast, and salt until evenly incorporated.
02 - Pour in lukewarm milk, softened butter, eggs, and vanilla extract; mix until a supple dough forms.
03 - Knead by hand or with a mixer for 8 to 10 minutes until the dough becomes smooth and elastic. Fold in 1/3 cup rainbow sprinkles until evenly distributed.
04 - Cover the bowl and set in a warm place for 1 hour or until the dough has doubled in volume.
05 - Heat 1 cup milk and split vanilla bean (or vanilla extract) in a saucepan until nearly simmering. In a separate bowl, whisk the egg yolks, sugar, and cornstarch until pale. Gradually pour the hot milk into the yolk mixture, whisking constantly. Return to the saucepan and cook over medium heat, stirring constantly, until thickened, about 2 minutes. Remove from heat, then stir in butter and cool fully.
06 - Transfer dough onto a floured surface and roll out to 1/2 inch thickness. Cut rounds using a 3-inch cookie cutter; arrange on a tray, cover, and let rise an additional 30 minutes.
07 - Heat oil in a deep pot to 340°F. Fry rounds in batches for 2 to 3 minutes per side, until deep golden and puffed. Drain on paper towels.
08 - Once cooled, use a piping bag fitted with a long tip to inject the vanilla cream into each donut.
09 - Dust donuts generously with powdered sugar and sprinkle with additional rainbow sprinkles. Serve immediately.