Preheat the oven to 350 degrees. Line a 9-inch-by-13-inch pan with parchment paper.

In a saucepan over medium heat, add beer. Stirring occasionally, let beer reduce until you have ½ cup, about 30 minutes.

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

In another bowl, using an electric mixer with a whisk attachment, mix the oil, eggs, sour cream, buttermilk and vanilla together until combined and aerated, 2 minutes on high.

Pour the wet ingredients into the dry ingredients, and combine. Add the reduced beer, and whisk until the batter is completely combined, 2 minutes.

Pour batter into the prepared pan. Bake for 30 to 35 minutes; the cake is done when an inserted toothpick comes out clean.

Remove the cake from the oven, and set on a wire rack. Allow to cool completely in the pan.

Combine the brown sugar, cream and beer in a medium saucepan, and heat to a simmer. Stirring frequently, let simmer until the sauce is reduced to 2 cups.

Remove from heat, and stir in the vanilla. Let cool slightly.

Melt white chocolate in a double boiler on the stove, or use the microwave. Set it aside to cool.

In a stand mixer with the whisk attachment, add butter. Beat on medium until smooth, 1 minute. Reduce speed to low, and slowly add in sugar.

Add cooled melted white chocolate, and increase speed back to medium. Beat until silky and smooth, 2 to 3 minutes.

Add heavy cream, vanilla and salt, and combine.

With the stand mixer on low, slowly add ½ cup of beer caramel. Mix until combined.

Level the cake if desired: Using a large serrated knife, slice a thin layer off the tops of the cakes to create a flat surface.

Spread buttercream over the cake. Drizzle the remaining caramel over the whole cake or slices of cake as desired.