Cup of Yum
1. Preheat oven to 350°F.
2. In a 2 cup or larger bowl or measuring cup, pour 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. You will be adding sour cream to this mixture after it cools, so use a bowl large enough to add sour cream.
3. In a medium bowl, add flour, baking powder and salt, gently stir with whisk to mix. Set aside.
4. In a large bowl of a stand mixer (or hand mixer), beat together butter and brown sugar on medium-high until well blended and creamy, about 2 minutes. Scrape down sides of bowl. Add egg and vanilla, beating until light and fluffy, about 2 more minutes, scraping sides.
5. Beat in half the flour mixture; scraping down the sides several times. Stir the sour cream into the cocoa mixture now cooled. 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.
6. Using a medium to large scoop, drop the dough onto parchment lined baking sheets, spacing about 3 inches apart. Don't make the rounds much larger or they may spread too much during baking.
7. 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.
8. Using spatula, transfer the cookies to wire racks. Cool completely.
9. Once cooled, 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 easier.

Marshmallow Filling

Cup of Yum
1. In the large bowl of a mixer, on medium speed, beat together the butter, powdered sugar, vanilla and salt until well blended and fluffy. Add the marshmallow cream and beat until evenly incorporated. 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.
2. 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. Shout WHOOPIE!!!!