Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.

In a large bowl, whisk together the flour, baking soda, salt, cocoa powder, and sugar.

In a separate bowl, whisk together the eggs, buttermilk, oil, and vanilla extract.

Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Do not overmix.

Gently stir in the Guinness stout until the batter is smooth.

Pour the batter evenly into the prepared cake pans.

Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.

Let the cakes cool in the pans for 10 minutes, then invert them onto a wire rack to cool completely.

In a large bowl, beat the softened butter until creamy.

Gradually add the powdered sugar and cocoa powder, alternating with the milk, beating until smooth and well combined.

Stir in the vanilla extract.

Once the cakes are completely cool, frost them with the buttercream frosting.