Preheat Oven to 375.

Line baking sheets with parchment paper.

Mix flour and baking soda in a small bowl, set aside.

Mix butter, peanut butter and brown sugar in a medium bowl until smooth.

Add egg and vanilla and beat until smooth.

Add flour mixture and stir until combined.

Roll dough into 24, 1 1/2 inch balls.

Place 2-3 inches apart on baking sheets and flatten slightly.

Bake 8-10 minutes. Remove when they look underdone if you like chewy cookies. Remove when barely brown at edges if you like them crumbly and crunchy.

Cool on a rack 10 minutes, then chill in freezer until completely cool (about 10 minutes).

While cookies cool, combine 2T butter and chocolate chips in a small bowl. Microwave until barely melted, about 1 minute. Stir well and let cool at room temperature until slightly stiffened.

Heat marshmallow cream in microwave safe bowl about 10 seconds to soften.

Spread one cookie with marshmallow cream and the other with the chocolate mixture. Press together until filling extends to cookie edges. Chill in a single layer until firm, then store in fridge.