• Using an electric mixer, mix softened (not melted!) butter, granulated sugar, and brown sugar together in a medium sized bowl.
• Mix egg yolks into butter and sugar mixture until combined.
• Mix in low-fat sour cream and vanilla extract.
• Sift together flour, baking powder, cornstarch, salt, and nutmeg in a larger bowl.
• Pour the wet ingredients into the dry ingredients, mixing with a spatula or with your hands until combined.
• Cover bowl and refrigerate dough overnight.

The Next Day

• Fill a saucepan with 2" of canola or vegetable oil and warm the oil over a medium-low heat until it reaches 350°F. Use a candy thermometer to test the temperature.
• Remove dough from the refrigerator and turn out onto a lightly floured surface. Using your hands or a lightly floured rolling pin, roll or pat the dough to ½" thickness. Use a doughnut cutter to cut out donuts. Role the doughnut hole scraps into smooth balls. Continue re-rolling and cutting to get as many doughnuts as possible from your dough.
• Place doughnut shapes and doughnut holes onto a lined baking sheet and place in the freezer for a few minutes until your oil is fully heated.
• Once oil has reached 350°F, remove baking sheet from the freezer and carefully drop doughnuts and doughnut holes into the oil, making sure not to crowd the pan.
• While your first batch of doughnuts is frying, stir together sugar and cinnamon in a shallow disk.
• Fry for 1 ½-2 minutes per side, using tongs to flip them in the oil, then remove from the pan and immediately coat them in the cinnamon sugar mixture. Then place on a paper towel lined plate to briefly drain.
• Between batches, wait a few minutes so that the oil temeprature comes back up to 350°F.
• Serve warm!