A Cronut is a croissant-doughnut pastry invented by New York City pastry chef Dominique Ansel.
Combine dry ingredients together: flours, sugar, salt and yeast. (If using active yeast; dissolve in some warm water or milk from the recipe). If using fresh yeast; crumble it over the liquid). In the mixer bowl, mix all ingredients together on low speed until combined. Increase speed to medium (2nd) and mix until dough will pull away from the sides of the bowl; scraping sides of the bowl as it goes. It should take 6 minutes. The internal dough temperature should be at 75/80ºF (25/27ºC). Make a tight ball and transfer dough in an oiled container, cover with a wet towel or plastic wrap and let rise in a warm area ≈75/85ºF/25/29ºC) for 90 minutes or until dough is doubled in size. Turn out on countertop and punch down the dough – shape into a tight oval, flatten, cover and freeze 15 min on each side; it slows down fermentation process – keep refrigerated for an hour or up to 12 hours prior lamination.
Give 3 single turn (Letter fold). The first turn gives 7 layers. The second turn gives 19 layers. The third turn gives 55 layers.
Roll out chilled dough into a ≈7''x22''https://bruno.b-cdn.net/18x55cm rectangle. Spread softened butter evenly to cover the entire surface of the dough. Fold like a business letter giving then the first single turn – Flatten a bit, cover and chill for 30 min to relax. On a floured work surface, roll out dough, seams set from noon to 6 into a ≈7x22''https://bruno.b-cdn.net/18x55cm rectangle. Fold like a business letter again giving the second single turn. Brush out any excess flour before folding. Repeat this step once. Wrap up and chill for 20 min to relax. Roll out pastry a bit and chill again for an hour. Flatten pastry into an even 16''x8.5''x0.5''https://bruno.b-cdn.net/42x22x1.25cm rectangle; cover and chill overnight. Cut out pastry into 8x3.5''https://bruno.b-cdn.net/9cm diameter rounds. Make a hole using a 1.2''https://bruno.b-cdn.net/3cm cookie cutter. Arrange cronuts on tray lined with a silicone mat. Save scraps for next batch or cut into samples to fry. Cover cronuts loosely with plastic wrap and let proof for about 90 minutes or until it reaches 1.4''/3.5cm height. For better handling, chill proofed cronuts 30 minutes prior frying.
Beat yolks, sugar, flour and corn starch together. Bring milk and vanilla to a boil and turn heat off. Temper yolk mixture with hot milk – pour into remaining hot milk; mix well. Turn heat back on and bring to boil; cook custard for 2 minutes whisking swiftly. Transfer custard onto a frozen baking tray lined with plastic wrap, cover with plastic wrap in contact; let cool and chill. Beat chilled custard prior using.
Mix lemon juice with powdered sugar.
Heat up 2 Qt/2L frying oil to 375ºF/190ºC. Fry proofed cronuts for a few minutes per side. Transfer on paper towels. Glaze cronuts lightly while still warm. Let cool and stuff with custard. Add zest if desired. Enjoy cronut within 3 hours!