Preheat your oven to 350°F and prepare your baking pan (typically 9x13 inches for 1.5 batches). Follow your gluten-free brownie recipe using King Arthur Measure for Measure flour—I've found this blend creates the best chewy texture that rivals traditional brownies. Mix your wet and dry ingredients according to your preferred brownie recipe, then pour into the prepared pan and bake for 25-30 minutes, until a toothpick inserted in the center comes out with just a few moist crumbs. Remove from the oven and let cool completely at room temperature for at least 30 minutes; this is crucial so the frosting doesn't melt into the warm brownies.

While the brownies cool, prepare your frosting. In a medium bowl, combine the 2/3 batch peanut butter frosting ingredients according to your recipe, using Jif creamy peanut butter for the smoothest consistency and best spreadability. Add the vanilla extract and mix until you reach a smooth, spreadable consistency—the frosting should be thick enough to hold its shape but soft enough to spread easily over the cooled brownies. If the frosting seems too stiff, loosen it with a teaspoon of milk or water at a time.

Once the brownies from Step 1 are completely cooled, spread the frosting mixture from Step 2 evenly over the top using an offset spatula or the back of a spoon. Work quickly but gently to avoid dragging up crumbs from the brownie layer. For the cleanest cuts and best texture, I like to chill the frosted brownies in the refrigerator for at least 30 minutes, or freeze for 15 minutes if you're short on time—this firms up the frosting and makes it easier to cut clean squares.

Remove the chilled brownies from the refrigerator or freezer. Using a sharp knife (dip it in hot water and wipe clean between cuts for neat edges), cut the brownies into squares of your preferred size. Serve immediately and enjoy—or store in an airtight container at room temperature for up to 3 days.