These are chocolate doughnuts in a thin shell of vanilla scented glaze. If you’ve never had a cake doughnut before you’re in for a treat. They’re light and rich, and certainly difficult to stop eating.
In large bowl whisk together the flour, cocoa powder, baking powder, and salt. Using an electric mixer fitted with a paddle attachment beat eggs until light in color and well combined, about a minute, add the sugar, butter, and buttermilk and beat until smooth. Add the egg mixture to the flour mixture and mix until just combined, do not overmix.
Add the flour mixture and continue to mix just until dough comes together.
Lightly flour a work surface and roll dough to 1/2-inch thick. Using a flour dipped 3-inch round cutter, cut out doughnuts. Use a flour dipped 1/2-inch round cutter to cut centers. (reserve centers for doughnut holes) Transfer cut doughnuts to a flour dusted baking sheet.
Top a cooling rack with a layer of paper towels, and place over a rimmed sheet tray. Attach a deep-frying thermometer to the side of large heavy-bottomed pot or dutch oven, heat 2-inches of oil, over medium-high, to 375°F Add doughnuts to the oil, being careful not to overcrowd, fry until doughnuts float to the surface of the oil, then continue to fry until golden about two minutes more. Turn doughnuts over and fry until the other side is golden, 30 to 60 seconds. Drain on paper towels.
In a small saucepan over low warm milk to a simmer. Pour warm milk into a medium heatproof bowl. Whisk in confectioners sugar, vanilla and salt until smooth and glossy. Place bowl over a pot of simmering water until thin and smooth, about 4 to 6 minutes. Remove from heat. Using your fingers, or tongs fully submerge each doughnut in the glaze. Place each glazed doughnut on a cooling rack placed over a baking sheet. Allow glaze to dry for at least 1 hour. Serve within a day.