In a stand mixer fitted with the hook attachment. Combine eggs, milk and yeast together. Add flours, sugar and salt. Mix on low to combine – increase speed to medium and mix for 15 min or until dough comes off the sides of the bowl and becomes elastic; scraping sides of bowl and hook as it goes. Add cubed butter and continue to mix for an additional 10 min; until completely smooth. The internal temperature of the dough should be at 77ºF/25ºC. Shape into round, and place in an oiled container. Cover and let ferment for about an hour or until it has doubled in size. Transfer dough onto work surface (no extra flour needed), degas, form a ball and refrigerate overnight; covered.
Turn the chilled dough out onto a floured surface and use a floured rolling pin to roll it to a 1/2''/13mm thickness. Using a cookie cutter, cut out 3.5''/9cm diameter rounds with 1''/2.6cm diameter holes. Or, to avoid trimmings*, divide dough into 2 oz/60g pieces, shape tightly into small balls and place them in a lightly floured tray and let rest 30 minutes loosely covered with plastic wrap. Flatten balls into 3.5''/9cm disks. Arrange each doughnut onto a 4x4''/10x10cm square and greased parchment, cover loosely with plastic wrap and let proof at room temperature until visibly puffy and airy, about an hour. Save trimmings for your next batch. If so, incorporate trimmings into the dough 5 minutes before the end of mixing process.
In a heavy-bottom large pot, Dutch oven or deep fryer heat 3 inches of oil until a deep-fry thermometer registers 360ºF/185ºC. Fry in both sides until golden brown. Coat in sugar if desired. Donut can be stuffed with preserves, and custard. Enjoy!