Texas Sheet Cake

Texas Sheet Cake
Ingredients
Cake
1 cup (2 sticks) unsalted butter, plus more for pan
½ cup (1¾ oz.) unsweetened natural cocoa powder (not Dutch-process)
3 Tbsp. canola oil
2 cups (250 g) all-purpose flour
2 cups (400 g) granulated sugar
1 tsp. baking soda
½ tsp. fine sea salt
2 large eggs
½ cup buttermilk
2 tsp. vanilla extract
Frosting and assembly
½ cup (1 stick) unsalted butter
¼ cup (1 oz.) unsweetened cocoa powder, preferably Dutch-process
⅓ cup whole milk
2 tsp. vanilla extract
3 cups (339 g) sifted powdered sugar
½ cup toasted chopped nuts (such as walnuts, pecans, or hazelnuts)
Preparation