Preheat the oven to 350°F and prepare 2 baking sheets with parchment. In a large mixing bowl, add the brown sugar, sour cream, vegetable oil, egg, and vanilla. Whisk until very smooth.

In a separate bowl, add flour, sifted cocoa powder, baking powder, baking soda and salt. Whisk until smooth.

Using a sieve, sift the dry ingredients into the bowl with the wet ingredients. Use a hand-mixer to mix the dry ingredients into the wet. Mix only until everything is incorporated and smooth, but ensuring not to over mix.

Using a small ¾ ounce cookie scoop, scoop little mounds of batter onto the prepared baking sheets (12 per sheet), ensuring there is about 2 inches of space between each.

Place the baking sheets into the oven and bake for 10-11 minutes. When done, the cookies should have a springy top, and a toothpick should come out clean. Allow to cool completely before filling.

In a medium mixing bowl, add the SunButter, butter and vanilla. Beat together with a hand mixer until well combined and smooth looking. Add the confectioners sugar, then beat until smooth.