Preheat your oven to 350°F and prepare an 8x8-inch or 9x9-inch baking pan by lining it with parchment paper or lightly greasing it. This ensures your bars won't stick and makes removal much easier once they're baked.

In a medium bowl, whisk together the gluten-free flour blend, gluten-free graham cracker crumbs, baking powder, and salt until evenly combined. This distributes the leavening agent throughout and prevents lumps in your final batter.

In a large bowl, beat the softened butter and packed brown sugar together for 2 minutes until light and fluffy—this helps incorporate air and creates a better texture. Add the egg and vanilla extract, mixing until fully combined and smooth. I find that room temperature butter makes this step much easier and ensures everything incorporates evenly without lumps.

Add the dry ingredient mixture from Step 2 to the wet ingredients from Step 3, folding and stirring until just combined. Don't overmix—a few streaks of flour are fine. Let the dough rest for 5 minutes; this allows the gluten-free flour to fully hydrate and makes the dough easier to work with.

Press half of the dough from Step 4 evenly into the bottom of your prepared pan. Spread the marshmallow creme in an even layer over the dough base. Sprinkle the melted dark chocolate chips over the marshmallow layer, spreading gently with a spatula or the back of a spoon. I like to melt the chocolate chips in the microwave in 20-second bursts, stirring between each one, so they're pourable but not scorched. Drop small scoops of the remaining dough over the chocolate layer—don't worry about covering everything completely, as the bars will have a rustic, textured top.

Place the pan in your preheated 350°F oven and bake for 32-35 minutes, until the top is golden brown and a toothpick inserted into the dough portions comes out with just a few moist crumbs. Remove from the oven and let the bars cool completely in the pan—this is important because they'll continue to set as they cool, making them easier to cut cleanly. Once fully cooled, use the parchment paper to lift the entire slab out of the pan and cut into squares.