Chocolate Sheet Cake, Gluten-Free

Ingredients

• 10 ounces gluten-free flours
• 2 teaspoons xanthan gum
• 1/2 teaspoon guar gum
• 2 cups sugar
• 1/4 teaspoon salt
• 1/2 cup buttermilk
• 2 large eggs
• 1 teaspoon vanilla extract
• 1 teaspoon baking soda
• 1/2 pound (2 sticks) butter
• 4 heaping tablespoons cocoa powder
• 1 3/4 sticks (7 ounces) butter
• 4 heaping tablespoons cocoa powder
• 6 tablespoons milk
• 1 teaspoon vanilla extract
• 1 pound (16 ounces) powdered sugar
• 1/2 cup finely chopped pecans