Preheat oven to 350 degrees. Spray a 9 x 13-inch baking pan with baking spray or grease with vegetable shortening.

In a large bowl, whisk together the flour, granulated sugar, brown sugar, and salt.

In a small saucepan, bring the butter, cocoa and cola to a boil over medium-high heat, stirring until smooth. Pour into the flour mixture and whisk until smooth.

In a small bowl, dissolve the baking soda in the buttermilk. Pour into the flour mixture and whisk until smooth. Whisk in the eggs and vanilla.

Pour the batter into the prepared pan and bake until a tester inserted in the center comes out clean, 40 to 45 minutes. Cool in the pan on a wire rack.

To make the frosting, use a stand mixer fitted with the paddle attachment, beat the butter on medium speed until smooth. Add the cocoa and cola and beat on low speed until smooth. Scrape the bowl. Beat in 1 cup of the powdered sugar on medium speed. Scrape the bowl. Beat in the remaining 1 cup powdered sugar on medium speed. Scrape the bowl. Spread over the cooled cake and add nuts if desired.

Cover the pan tightly and store at room temperature for up to 2 days.