01 -
Combine softened butter, granulated sugar, and brown sugar in a large bowl. Use an electric mixer to beat until light and fluffy.
02 -
Add egg yolks to the creamed mixture and beat until fully combined.
03 -
Stir in vanilla extract and sour cream, scraping the sides and bottom of the bowl to incorporate all ingredients evenly.
04 -
In a separate bowl, whisk together all-purpose flour, baking powder, cornstarch, salt, and nutmeg if using.
05 -
Add the dry ingredients to the wet mixture and gently fold by hand using a spatula until just combined, avoiding over-mixing to prevent toughness.
06 -
Cover the bowl with plastic wrap and refrigerate the dough for at least one hour to firm up.
07 -
Turn the chilled dough onto a lightly floured surface. Roll out to approximately 1.27 cm (1/2 inch) thickness and cut into shapes using a donut cutter. Save the holes for frying.
08 -
Gather scraps, reroll, and cut additional donuts to maximize yield.
09 -
Place the cut donuts and holes onto a wax paper-lined baking sheet and refrigerate for at least 15 minutes while heating oil.
10 -
Fill a Dutch oven or deep heavy-bottomed pot with 5 to 7.5 cm (2 to 3 inches) of vegetable or canola oil. Attach a candy thermometer ensuring it does not touch the bottom. Heat over medium-low until the oil reaches 175–185°C (350–360°F).
11 -
In a small bowl, whisk melted butter, powdered sugar, vanilla, and hot water until smooth. Set aside.
12 -
Using tongs, carefully lower 3 to 4 donuts or holes into the hot oil. Maintain oil temperature by allowing recovery between batches. Fry donuts for 1 to 1.5 minutes per side until golden brown.
13 -
Remove fried donuts to a paper towel-lined plate and let cool for 1 to 5 minutes.
14 -
Dip each warm donut into the prepared glaze, then transfer to a cooling rack to set.
15 -
Enjoy donuts warm for optimal flavor and texture. Store leftovers in an airtight container at room temperature for several days.