Preparation:
1. Cake
Step 1
Place a rack in center of oven; preheat oven to 375°F. Grease 15x10x2″ baking pan with unsalted butter.
Step 2
Melt 1 cup (2 sticks) unsalted butter in a large saucepan over medium heat. Whisk in ½ cup (1¾ oz.) cocoa powder. Add 3 Tbsp. canola oil and 1 cup water and bring to a boil; boil for 30 seconds. Let cool slightly.
Step 3
Meanwhile, sift together 2 cups (250 g) all-purpose flour, 2 cups (400 g) granulated sugar, 1 tsp. baking soda, and ½ tsp. fine sea salt into a large bowl, then whisk to combine. Pour warm cocoa mixture into flour mixture and whisk until just combined.
Step 4
In a small bowl, whisk together 2 large eggs, ½ cup buttermilk, and 2 tsp. vanilla extract. With a rubber spatula, stir egg mixture into batter. Pour cake batter into greased pan. Bake cake until top is firm and a toothpick or cake tester inserted in the center comes out with moist crumbs attached, 32–35 minutes.
2. Frosting and assembly
Step 5
Melt ½ cup (1 stick) unsalted butter in a medium saucepan over medium heat. Whisk in ¼ cup (1 oz.) cocoa powder, preferably Dutch process, and bring to a boil; boil for 30 seconds. Remove from heat and whisk in ⅓ cup whole milk and 2 tsp. pure vanilla extract. Add 3 cups (339 g) sifted powdered sugar 1 cup at a time while whisking until combined.
Step 6
Immediately after cake comes out of oven, pour frosting over. Sprinkle with ½ cup toasted chopped nuts (such as walnuts, pecans, or hazelnuts). Try not to jiggle the cake before it sets or there will be waves in the frosting. Let cool before cutting into squares.
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