Step 1: Preheat your oven to 350°F (175°C). Grease or line an 8x8-inch baking pan with parchment paper, leaving an overhang on the sides.
Step 2: In a large bowl, combine the flour, granulated sugar, baking powder, and salt. Add the softened butter, egg, and 1 tsp vanilla extract. Stir until a soft dough forms. Press the dough evenly into the bottom of the prepared pan.
Step 3: In a small bowl, combine the brown sugar, ground cinnamon, and 2 tbsp melted butter. Stir well until a smooth paste forms. Drizzle and spread this cinnamon-sugar mixture evenly over the dough.
Step 4: Use a knife or a toothpick to gently swirl the cinnamon-sugar mixture into the dough, creating a marbled effect. Be careful not to overmix.
Step 5: Bake for 25-30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out clean.
Step 6: While the bars are baking, prepare the glaze. In a small bowl, whisk together the powdered sugar, 1-2 tbsp milk (adjust for desired consistency), and 1/2 tsp vanilla extract until smooth.
Step 7: Remove the bars from the oven and let them cool in the pan for 10 minutes. Transfer to a wire rack to cool completely. Once cool, drizzle the glaze over the top before slicing and serving.
Step 8: Store any leftover bars in an airtight container at room temperature for up to 3-4 days.