Preheat & Prep: Preheat your oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, letting the edges hang over for easy removal.
Mix the Blondie Batter: In a large bowl, whisk together the melted butter and brown sugar until smooth. Add the egg and vanilla extract, and whisk again until fully combined.
Combine the Dry Ingredients: In a separate bowl, mix together the gluten-free flour, baking powder, and salt. Add the dry ingredients to the wet mixture, stirring until just combined.
Spread & Swirl: Pour the batter into the prepared baking pan and smooth it out with a spatula. Now, mix together the melted butter, granulated sugar, and cinnamon for your swirl. Drop spoonfuls of the cinnamon mixture over the blondie batter, then use a knife or toothpick to swirl it through.
Bake: Bake for 22-26 minutes, or until a toothpick inserted into the center comes out with just a few crumbs. Remember, a little underbaked means extra gooey. Let them cool in the pan for about 10 minutes, then lift them out using the parchment paper and transfer to a wire rack to cool completely.
Make the Glaze: While the blondies cool, whisk together the powdered sugar, milk, and vanilla extract until smooth. It should be thick but pourable—add a bit more milk if needed. Drizzle the glaze over the cooled blondies.
Slice & Serve: Cut the blondies into squares (or bars, no judgment here). Enjoy as-is, or warm them up for that fresh-out-of-the-oven vibe. Pair with a hot coffee or a big glass of milk for maximum cozy feels.