Step 1

Make the donuts: Place 4 tablespoons unsalted butter in a small microwave-safe bowl. Microwave on HIGH until melted. Set aside to cool slightly.

Step 2

Place 3 1/2 cups cake flour, 2 3/4 teaspoons baking powder, 1/4 teaspoon baking soda, 1 1/2 teaspoons kosher salt and 3/4 teaspoon ground nutmeg in a medium bowl and whisk to combine.

Step 3

Place the melted butter, 1/2 cup packed light brown sugar, 1/2 cup granulated sugar, and 1 tablespoon vanilla extract in the bowl of a stand mixer fitted with the paddle attachment. (Alternatively, use a large bowl and electric handheld mixer.) Beat on medium speed until fluffy, scraping the bowl with a rubber spatula as needed, about 2 minutes.

Step 4

Reduce the speed to low. Add 1 large egg and 3 large egg yolks one at a time, stopping and scraping down the bowl as needed. Add 3/4 cup buttermilk and mix until combined.

Step 5

Add the dry ingredients all at once and mix just until a few streaks of flour remain. Remove the bowl and continue mixing by hand until there are no dry bits of flour. The dough will be quite sticky. Refrigerate at least 1 hour and up to overnight. The rest in the fridge makes the dough easier to work with: it remains sticky, but firms up considerably.

Step 6

Line two rimmed baking sheets with parchment paper. Heavily flour a work surface and your hands with cake flour. Transfer half of the dough to the work surface, and sprinkle it with flour. Pat the dough into a 3x24-inch rectangle with the long side facing you, adding more flour to the work surface as needed. The dough is extremely soft and sticky, so don’t be afraid to use as much flour as you need.

Step 7

Use a floured bench scraper or butter knife to cut the rectangle into 1 1/2-inch wide bars, flouring underneath the bars, the work surface, and the bench scraper as you work. Once all the bars are cut, dip the bench scraper in flour and very gently press it down the center of each bar, making a slight indentation (this will give your donut the quintessential buttermilk bar look).

Step 8

Using a brush, lightly brush the excess flour off the bars. Use a floured bench scraper or thin metal spatula to gently transfer each bar to one of the prepared sheets, flipping it over so that it is indentation-side down. Brush the excess flour from the other side of the bars. Refrigerate and repeat with shaping and cutting the remaining dough.

Step 9

Fill a large, heavy pot or Dutch oven with at least 2 inches of vegetable oil. Heat the oil on medium-high heat until 350ºF, or a bit above (the temperature will drop when you add your donuts). Meanwhile, fit a wire rack over a baking sheet.

Step 10

Remove the first tray of bars from the refrigerator. Frying in batches of 4 to 5 so as not to crowd the pot, use a thin metal spatula to carefully transfer the donuts into the hot oil. Fry until dark golden brown, about 1 minute per side. Carefully transfer the donuts to the rack with the spatula. Repeat frying the remaining donuts. Let cool to room temperature before glazing.

Step 11

Make the glaze: Place 2 cups powdered sugar in a large bowl. Add 1 tablespoon vanilla extract and 1/4 cup boiling water and whisk until shiny and smooth. Dip a donut indentation-side down halfway into the glaze. Lift it up and allow the excess glaze to drip off. Return to the rack glazed-side up and repeat with the remaining donuts.