Spray or oil the bundt cake pan. Sprinkle with a little flour or cacao powder. Dust any excess off and set aside.
Preheat oven to 350.
In a medium bowl sift together flour, cacao powder, baking soda, baking powder, and salt. Set aside.
In a large mixing bowl, use a mixer to cream together the butter and sugar.
Add the eggs and the milk. Mix until completely incorporated.
With a rubber spatula mix the dry ingredients 1/2 at a time into the wet. (I like to do this so the mixer doesn't cause the flour to get everywhere.)
After you mix it together with the spatula. With your mixer on low pour in the 1/4 cup of boiling water and 3/4 cup of Guinness. Add the vanilla extract. Mix until smooth on medium speed about 1-2 minutes,
Pour cake batter into the oiled cake pan and bake for about 45-55 minutes or until a knife inserted comes out clean.
Let the cake cool for a bit, then remove by running a rubber spatula along the edges to loosen.