Preheat your oven to 350°F (175°C).

Grease a 9x13 inch baking pan with butter or cooking spray.

Mix the dry ingredients in a large bowl: flour, cocoa powder, sugar, baking soda, and salt. Whisk until combined and free of lumps.

Combine the wet ingredients in another bowl: eggs, buttermilk, and vanilla extract. Stir well until blended.

Pour the Guinness into the wet mix gradually, stirring gently.

Fold the dry ingredients into the wet mixture until just combined.

Transfer the batter to your prepared pan, smoothing out the top.

Bake in your preheated oven for 30-35 minutes or until a toothpick inserted comes out clean.

Cool the cake in the pan for 10 minutes before transferring it to a wire rack to cool completely.

In a mixing bowl, beat the cream cheese until smooth.

Gradually add powdered sugar and beat until well combined.

Add vanilla extract and heavy cream, then mix until fluffy and spreadable.

Frost the cooled cake with the cream cheese frosting.