In a medium bowl, whisk the flour, cocoa powder, espresso powder, baking powder, and salt together, set aside.
Add the butter to the bowl of a stand mixer fitted with the paddle blade (or, better, a scraper blade) and beat on medium speed until pale and fluffy. Add the egg yolks and sugar, and continue beating until completely incorporated, and the sugar is dissolved.
Reduce speed to low and slowly add the dry ingredients. Mix only until incorporated (do not over mix).
Turn out the dough on cutting board, gather up into a rough mass, and then cut the dough in half. Form each half into a thick log, and wrap each in plastic wrap. Freeze the dough for about 30 minutes, until firm (but not frozen solid).
Preheat oven to 350°F with rack in the center position.
Remove one log of dough from the freezer and grate the dough on the large-hold side of a box grater (or use the grating blade on a food processor) into a 7" square cake ring or 8" springform pan.
Gently pat the dough shreds to even it out in the pan, pushing the shreds into the corners.
Spread the marshmallow creme over the top, using an offset spatula. Sprinkle the chocolate chips over the marshmallow filling.
Grate the second ball of dough over the filling, and pat to even out the surface.
Bake for 40 minutes. The shortbread will probably have puffed up from the marshmallow filling - that's okay, it will settle down as it cools. Remove from oven, and immediately dust with confectioner's sugar through a sifter or small sieve. Allow the shortbread to cool in the pan, set on a rack. Cut into squares or triangles, and serve. Store, covered, at room temperature.
*We're replacing part of the flour that normally goes with a shortbread recipe with the cocoa powder. You can measure it out how ever it makes sense to you, but I find it easiest to just scoop out 2 tablespoons from one of the cup measurements.For the easy grating, a food processor with a grating disk does a great job here. Form the dough into 4 logs that are skinnier than the feed chute for your food processor. Wrap each with cling wrap and place in the freezer for no more than an hour. You don't want the logs to freeze solid; they might damage your food processor.
Chocolate Marshmallow Shortbread Bars was originally published on https://soupaddict.com/chocolate-marshmallow-shortbread-bars/