1. 1. Prepare Your Baking Pan and Oven: Start by preheating your oven to 350 F (175 C). Lightly grease and flour an 8×8 inch baking pan, or line it with parchment paper, leaving an overhang on two sides to easily lift the bars out later. This ensures your bars won’t stick and makes for easy cleanup.

2. 2. Combine Dry Ingredients: In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Make sure these are well combined so the leavening agents are evenly distributed throughout the batter. Set this bowl aside.

3. 3. Cream Wet Ingredients: In a large bowl, using an electric mixer or a sturdy whisk, cream together the softened unsalted butter and granulated sugar until the mixture is light and fluffy. This usually takes about 2-3 minutes. Beat in the large egg until fully incorporated, then stir in the sourdough discard and vanilla extract until just combined.

4. 4. Alternate Wet and Dry: Gradually add the dry ingredient mixture to the wet ingredients, alternating with the buttermilk. Begin and end with the dry ingredients. Mix on low speed or by hand until just combined. Be careful not to overmix; a few lumps are okay. Overmixing can lead to tough bars.

5. 5. Assemble the Swirl: Pour about two-thirds of the batter into your prepared baking pan and spread it evenly. In a small bowl, combine the brown sugar and ground cinnamon for the swirl. Sprinkle half of this cinnamon sugar mixture evenly over the batter in the pan. Dollop the remaining one-third of the batter over the cinnamon sugar layer, then carefully spread it out. Sprinkle the remaining cinnamon sugar mixture over the top. Drizzle the 2 tablespoons of melted butter over the top of the cinnamon sugar.

6. 6. Bake to Perfection: Place the baking pan in the preheated oven and bake for 25-30 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. The top should be golden brown and the edges slightly pulled away from the pan.

7. 7. Cool and Glaze: Once baked, remove the pan from the oven and let the coffee cake bars cool in the pan on a wire rack for at least 15 minutes. While they cool, whisk together the powdered sugar, milk, and vanilla extract in a small bowl until smooth to make the optional glaze. Once the bars have cooled slightly, drizzle the glaze evenly over the top.

8. 8. Slice and Serve: Allow the glaze to set for a few minutes before lifting the bars out of the pan using the parchment paper overhang. Slice into squares and serve warm or at room temperature.