In a large bowl, combine milk and yeast.
Add sugar, eggs, vanilla, and salt.
Add one cup of the flour. Mix well.
Add two more cups of flour, kneading until dough is elastic but still a little tacky.
Transfer the cronut dough to a baking sheet. Cover with plastic wrap and chill in the refrigerator for half an hour.
While dough chills, combine the butter and 1/4 cup flour with mixer, until smooth.
Remove dough from and set it onto the lightly floured counter top.
Roll the dough into a rectangle about 1/2″ thick.
Spread the butter evenly over the dough.
Fold the dough in thirds.
Cover the dough in plastic wrap, and put it back in the refrigerator for another 30 minutes.
(Take the dough out, put it back on the countertop, and roll it out into another rectangle. Be sure to keep the open sides to your left and right. Fold the dough into thirds again. Chill the dough for another 30 minutes.) Repeat FOUR TIMES, leaving the wrapped dough to chill for an hour at the end of the fourth time.
Heat deep fryer to 350* F.
Roll your dough out to 1″ thickness, then cut into rings.
Fry croissant rings, one at a time - a minute or two on each side. Flip as necessary until they’re the perfect golden brown.
Set croissant rings on paper towels to drain.
Roll in cinnamon and sugar, and cover with maple glaze or the frosting/icing of your choice.
(To make Maple Glaze - Whisk together powdered sugar, maple syrup, and milk. Drizzle over cronuts while they’re still warm.)
Serve and Enjoy!