Cook the diced apple with 1 tablespoon butter and cinnamon in a small skillet for about 5 minutes. Transfer it to a plate and allow it to cool.
Preheat the oven to 350°F or 175°C. Line an 8-inch square pan with parchment paper, leaving enough overhang to lift out the blondies.
Whisk melted butter and brown sugar until smooth. Add the egg and vanilla, then whisk again until the mixture looks glossy.
Stir in flour, baking powder, and salt just until combined. Fold in the cooled apple pieces without overworking the batter.
Spread the mixture evenly in the pan and smooth the top. Bake for 27 to 32 minutes, until the edges are set and the center no longer looks wet.
Cool completely before removing the slab from the pan. Cut with a sharp knife, wiping the blade between slices for cleaner edges.
