Preheat oven to 350ºF and line two baking sheets with nonstick silicon mats or parchment paper. Set aside.
In the bowl of a stand mixer, combine butter and brown sugar and whip on high speed until light and fluffy.
Add egg and vanilla, and beat to combine.
In a separate large mixing bowl, combine flour, cocoa powder, baking soda, espresso powder and salt and whisk to combine.
With the mixer on a low speed, add the dry ingredients to the wet ingredients, alternating with the buttermilk until just combined.
Use a medium cookie scoop to portion dough onto prepare baking sheets. You want to make sure you're leaving enough room (about 2") between each cake so they can spread. I wouldn't do more than 5 on a baking sheet at a time, just to be safe.
Bake for 9-11 minutes, or until the tops spring back when touched.
Let cool on baking sheets for 5 minutes, then move to wire racks to cool completely.