Preheat your oven to 350°F. Grease your cake pan with butter and line with parchment paper.
In a small sauce pan over medium-low heat, melt the 10 tablespoons of butter with the 1 cup of beer. Once the butter is completely melted, remove from the heat and add in the cocoa powder. Whisk together until completely combined, and then add the brown and granulated sugar. Whisk until no lumps remain.
In a bowl, combine the eggs, sour cream and vanilla extract with a mixer until creamy and smooth, about one minute. Pour in the chocolate and beer mixture. Whisk together until well combined. Add in the flour and baking soda. Mix on low speed until well incorporated, about 30 seconds to 1 minute.
Pour cake batter into prepared pan evenly. Bake the cake for 1 hour. The cake might take even longer, so be sure to insert a knife in the center to see if it comes out clean. If it doesn't, continue baking in 5 minute increments. Leave out to cool.
While the cake is baking, make the frosting. In a bowl, beat the butter on medium until it is fluffy and whipped, about one minute. Add in the Irish cream liqueur and the confectioner's sugar. Whip together on medium until fully combined and no lumps remain.
Do not frost the cake until it has cooled ENTIRELY, about 30-45 minutes. Then remove the cake from the pan and frost it with the buttercream. The frosting will only sit on the top, not the sides.
Slice and enjoy!