To a large mixing bowl, add the melted room temperature butter with your sugar. Using a whisk, mix well. Then add your buttermilk, Greek yogurt, flax eggs, peppermint and vanilla extracts. Whisk well to combine.
To the same bowl, add the flour cocoa powder, baking soda, baking powder, cornstarch, and salt. Using a whisk, gently whisk the dry ingredients into the wet ingredients.
Add half of the room temperature coffee to the mixture, gently whisk, and then add the rest of the coffee. Finish whisking until just combined (avoid overmixing your batter).