|unsweetened (alkalized) cocoa powder|
|pure vanilla extract||.||.|
|bleached all-purpose flour|
|unsalted butter (65° to 75°F/19° to 23°C)|
Twenty minutes or more before baking, set an oven rack in the lower third of the oven and
Spread the walnuts evenly on a baking sheet and bake for about 7 minutes to enhance the flavor. Stir once or twice to ensure even toasting and avoid overbrowning. Turn the walnuts out onto a clean dish towel and roll and rub them around to loosen the skins. Discard the skins and cool completely. In a food processor, pulse them until they are medium fine. If they start to become a little pasty, add ½ cup of the flour. (If you don’t like the texture of nuts in cake, if chopped fine you’ll have the enhanced chocolate flavor and no perception of nuts.)
In a medium bowl, stir, then whisk the sour cream, cocoa, eggs, and vanilla until the consistency of slightly lumpy muffin batter.
In the bowl of a stand mixer fitted with the flat beater, mix the ground walnuts, flour, sugar, baking powder, baking soda, and salt on low speed for about 30 seconds. Add the butter and half the cocoa mixture. Mix on low speed until the dry ingredients are moistened. Raise the speed to medium and beat for 1½ minutes. Scrape down the sides of the bowl. With the mixer off between additions, add the remaining cocoa mixture in two parts, starting on medium-low speed and gradually raising the speed to medium. Beat on medium speed for 30 seconds after each addition to incorporate the ingredients and strengthen the structure. Scrape down the sides of the bowl. Using a silicone spatula or spoon, scrape the batter into the prepared pan and smooth the surface evenly with a small metal spatula.
Make the cocoa syrup shortly before the cake is finished baking.
As soon as the cake comes out of the oven, place the pan on a wire rack, poke the cake all over with a wooden skewer, and brush it with about one-third of the syrup. Let the cake cool in the pan for 10 minutes. Drape plastic wrap over the cake in the pan, overhanging the pan by a few inches. Place a 9-inch cardboard round or plate on top of the plastic wrap and invert the cake. Remove the pan and flatten the plastic wrap overhang onto the work surface. Brush the top and sides of the cake with the remaining syrup. Bring up the sides of the plastic wrap to apply any little puddles of syrup to the cake. Cool completely. When ready to serve, invert the cake onto another cardboard round or plate lined with plastic wrap. Gently remove the plastic wrap sticking to the cake, being careful not to tear or break off any of the fragile edges of the cake, and reinvert the cake onto a serving plate.