Preheat oven to 350°. In a 2 cup or larger bowl or measuring cup, pour in hot water, stir in cocoa powder and baking soda with a fork until smooth; the mixture will foam up (a lot) and darken. Let the mixture stand until the foaming subsides and the mixture has cooled down. NOTE: you will be adding sour cream to this mixture after it cools, use a large enough bowl or cup.
In a medium bowl, place all purpose flour, baking powder and salt, gently stir with whisk to mix. Set aside.
In a large bowl of an electric mixer, beat together the butter and brown sugar on medium-high until well blended and creamy, about 2 minutes. Scrape down sides of bowl. Add the egg and vanilla and beat until light and fluffy, about 2 more minutes, scraping sides often.
Beat in half of the flour mixture; scraping down the sides several times. Stir the sour cream into the cooled cocoa mixture. On low speed, beat the cocoa mixture and the remaining flour mixture, into the brown sugar mixture until even incorporated, Scraping down sides until you no longer see any of the white flour or creamed mixture.
Using a medium to large scoop, drop the dough onto parchment lined baking sheets, spacing about 3 inches apart. As these are a sandwich cookie, try and make an even number of cookies so they match up.
Bake the cookies at 350, one sheet at a time, in the middle of the oven for about 10-12 minutes, or until they spring back when lightly pressed in the center. Transfer the sheet to a wire rack and let stand until cookies firm up, 4-5 minutes. Using spatula, transfer the cookies to wire racks. Let stand until completely cool.
After cooling, the cookies may be placed in freezer storage bags, layer using wax paper in between. Lay flat in the freezer. You may fill them straight from the freezer, sometimes frosting them is easier.
Marshmallow Fluffernutter Buttercream
Whip your butter until light and fluffy, about 2 minutes. Then add marshmallow cream, peanut butter, salt and vanilla, mixing until combined. Add powdered sugar and milk alternately on low speed. Once incorporated, increase speed to medium-high and beat until light and fluffy, about 4 minutes.
Using a medium or large scoop, plop a generous portion of frosting onto "bottom" of cookie, leaving a ½ " border around the edge. Top with matching cookie top. Pressing gently to sandwich them together, just until filling reaches edges.
Store cookies in individual plastic bags or wrap in plastic wrap. Refrigerate or freeze (great to put in a lunch box) until ready to enjoy. WHOOPIE!!!!
Notes
TIP | If the frosting is too soft or runny, add a little more powdered sugar until it thickens & scoopable.