1 Preheat the oven to 350°F, and prepare your cake pan by rubbing butter all over the sides and bottom. Then, cut a circular piece of parchment to place in the very bottom of the cake pan.
2 In a large bowl, whisk together the dry ingredients.
3 Prepare a double boiler by adding 2-inches of water in a medium saucepan over high heat. Bring to a simmer, then reduce the heat to keep the water at a slow simmer. Add a large glass or metal bowl over top, making sure the fit is snug and the bottom of the bowl does not touch the water.
4 To the large glass bowl, whisk the butter and buttermilk together over the heat until the butter is totally melted. Add the espresso powder (or instant coffee), eggs, and vanilla. Whisk to combine.
5 Pour the wet, combined ingredients into the bowl with the dry ingredients and whisk until smooth. Stir in chocolate chips.
6 Pour the batter into the prepared cake pan and bake for 35-45 minutes until a knife comes out (from the center) clean.
9 While the cake is in the oven, make the frosting. To do this, add all of the ingredients to a stand mixer with a whisk attachment, and, starting on low speed, mix the ingredients until combined. Then, increase the speed to medium-high and mix until smooth.
7 Let the cake cool for 30 minutes in the pan, then transfer to a wire rack (run knife along sides and then flip over to remove from pan).
8 Let the cake cool completely on the wire rack, then ice or frost as desired.
