¾ cup (64 grams) Dutch process cocoa powder (see Note)
½ teaspoon (2.5 grams) baking powder
¼ teaspoon (1.25 grams) baking soda
¼ teaspoon kosher salt
Buttermilk Glaze (recipe follows)
Instructions
Preheat oven to 325°F (170°C). Butter and flour a 9x5-inch loaf pan.
In the bowl of a stand mixer fitted with the paddle attachment, beat butter and sugars at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add egg, beating well. Add sourdough discard, buttermilk, and vanilla, beating until combined.
In a medium bowl, whisk together flour, cocoa, baking powder, baking soda, and salt. (If your cocoa is very lumpy, you can sift it in.) With mixer on low speed, gradually add flour mixture to butter mixture, beating until combined. Give batter a quick fold with a spatula to ensure bottom of bowl doesn’t have any unmixed ingredients. Pour batter into prepared pan.
Bake until a wooden pick inserted in center comes out clean, 1 hour to 1 hour and 5 minutes. Let cool in pan for 10 to 15 minutes. Remove from pan, and let cool completely on a wire rack. Wrap tightly in plastic wrap, and store at room temperature for up to 5 days. Spoon Buttermilk Glaze over cake before serving.
Notes
It is important to use Dutch process cocoa powder in this recipe. Natural process cocoa powder is much more acidic, and the cake will be both lighter in color and a bit bitter if you substitute. Droste is my favorite Dutch process cocoa powder.
Recipe by Bake from Scratch at https://staging.bakefromscratch.com/sourdough-chocolate-loaf-cake/