Grease 2 (8-inch) round cake pans. Line with parchment paper cutting circles to fit the bottom of each pan. Preheat oven to 350 degrees F.
Combine the Guinness and butter in a medium saucepan over medium heat. Bring to a low simmer. Whisk in cocoa powder. Remove from heat and cool.
In a medium-sized bowl combine the flour, sugar, baking soda and salt. With a whisk, blend until uniform.
In a free-standing mixer fitted with a paddle attachment (or you can use a hand mixer) mix the eggs and sour cream on medium speed. Add the Guinness mixture and mix until uniform.
Slowly add the flour and blend until uniform, scraping down the sides as needed. Do not over-mix cake batter.
Divide the batter between the 2 prepared cake pans. Bake about 30 minutes or until a toothpick inserted to the center of the cake comes out clean. Remove to cooling rack and cool completely.
Remove cakes from pans by running a knife around the edges. If the cake does not come out when inverted, run the cake pan over the burner of your stove on low heat making a circular motion with the pan. Make sure you do not burn the cake, or your hands. Use oven mitts. Just a few seconds should be enough. The cake should easily come out of the pan when inverted. Remove the parchment paper.
While the cake is cooling make the frosting by combining the butter and cream cheese in a mixer fitted with a paddle attachment. Mix until uniform.
Add the Dark Chocolate Toffee Sauce and vanilla extract, mix until combined.
Slowly add the confectioner's sugar and mix on low speed scraping down sides as needed. Add sugar to desired consistency and/or chill in the refrigerator. Frost cake and top with Heath Bar Crunch.