Preheat the oven to 350F. Line an 8 x 8-inch square pan with parchment paper.
In a stand mixer fitted with the paddle attachment, beat together the coconut sugar, coconut oil, and vanilla on high speed for 1 minute.
In a medium mixing bowl, whisk together the oat flour, baking powder, and sea salt. Place within reach of the mixer.
With the mixer off, add the water to the coconut sugar mixture and start beating while slowly adding the oat flour mixture. Beat for 30 seconds, or until the dough pulls together into large, moist mounds as it moves around the mixing bowl. When you first add the dry ingredients, the dough may be dry and crumbly; however, just keep beating on high—the finished dough will look and feel similar to Play-Doh. If the dough is still dry and crumbly after beating for 30 seconds, add more filtered water, 1 tablespoon at a time, and continue beating until it pulls together.
Divide the dough in half. Press half of the dough into the bottom of the lined pan. Then, grasp the edges of the parchment paper and lift out pressed dough. Set aside.
Line the pan with a fresh sheet of parchment paper. Press the remaining dough evenly into the bottom of the pan. Top with the chocolate bars, breaking or chopping into pieces if needed to evenly cover the surface. Then, top with the mini marshmallows, evenly spreading them over the surface of the chocolate. Finally, very carefully top with the reserved cookie dough layer, removing the parchment paper and then gently pressing and compacting on top of the marshmallow layer.
Bake for 18 to 22 minutes, or until the top layer of dough is golden brown and just beginning to show soft cracks across the surface. Cool completely**.
Once completely cool, slice into bars*** and serve.
Store in an airtight container at room temperature for up to 3 days or in the refrigerator for up to 1 week.