¼ cup unsalted butter, room temperature, plus more
1½ cups all-purpose flour
½ cup unsweetened cocoa powder
1 teaspoon kosher salt
¾ teaspoon baking powder
½ cup virgin coconut oil, room temperature
1½ cups plus 1 tablespoon sugar
3 large eggs
1 teaspoon vanilla extract
⅔ cup buttermilk
¼ cup unsweetened coconut flakes
Preheat oven to 325°. Butter an 8×4” loaf pan; line with parchment paper, leaving a generous overhang on long sides. Whisk flour, cocoa powder, salt, and baking powder in a medium bowl; set aside.
Using an electric mixer on medium-high speed, beat oil, ¼ cup butter, and 1½ cups sugar until pale and fluffy, 5–7 minutes. Add eggs one at a time, beating to blend between additions; beat until mixture is very light and doubled in volume, 5–8 minutes. Add vanilla.
Reduce mixer speed to low and add dry ingredients in 3 additions, alternating with buttermilk in 2 additions, beginning and ending with dry ingredients (do not overmix; it will cause cake to buckle and split). Scrape batter into prepared pan and run a spatula through the center, creating a canal. Sprinkle with coconut and remaining 1 Tbsp. sugar.
Bake cake, tenting with foil if coconut browns too much before cake is done (it should be very dark and toasted), until a tester inserted into the center comes out clean, 70–80 minutes. Transfer pan to a wire rack; let cake cool in pan 20 minutes before turning out.
Cake can be baked 5 days ahead. Keep tightly wrapped at room temperature.