Grease a 9x13 baking pan and dust with cocoa powder. Set aside. Preheat the oven to 350°F.
Whisk together the oil, buttermilk, sour cream, espresso, both sugars, and eggs together until well-combined.
Mix together the flour, cocoa powder, baking soda, and salt together until well-combined in a separate bowl.
Mix together the wet and dry ingredients together. Mix just until there are no streaks of flour. Do not overmix, having some small lumps in the batter is okay.
Pour the batter into the prepared baking pan.
Bake for about 22 - 25 minutes, or until a toothpick comes out clean.
Start making the frosting as the cake is baking.
Melt the chocolate and set aside to cool slightly.
Beat the butter until smooth. Add in the powdered sugar and cocoa powder, mix well. The mixture may look lumpy, that is okay.
Stir in the vanilla and salt.
Gently drizzle in the melted chocolate and mix until well-combined.
Add in the heavy cream one tablespoon at at time until the frosting reaches your desired consistency.
Allow the baked cake to cool fully, then spread with the frosting.
Notes
When measuring ingredients, especially the flour and powdered sugar, fluff the ingredient, then spoon into your measuring cup. Gently level off with a knife.
You can use any neutral-tasting oil in the cake.
Any brown sugar will work.
Any chocolate will work, but you will get the best results with a darker chocolate and using chopped bars instead of chips.