1. Preheat the oven to 350°F. Spray a 9-inch square baking pan with cooking spray and line it with parchment paper, leaving some parchment paper on two sides to help remove the bars from the pan. Set the prepared baking pan aside.

2. Cut off and discard the ends from one lemon. Quarter the lemon and remove seeds. Transfer to the food processor and process until finely chopped, about 1 minute. Measure out 1/3 cup of the pulp and discard any extra, or make another pan of these bars Return the reserved 1/3 cup pulp to the processor.

3. Add the granulated sugar, brown sugar, melted butter, and large egg. Process sugars until smooth and well combined, about 15 seconds.

Add the flour, baking powder, and salt. Pulse the dry ingredients until just incorporated, about 5 pulses. Scrape down the sides as needed. Watch for streaks of flour and stop as soon as they disappear. The blondie batter will be thick. Transfer it to the prepared baking pan and use a rubber spatula or an offset spatula to spread it into an even layer.

4. Bake tthe bars until the edges are set and golden brown and a wooden pick inserted into the center comes out with a few moist crumbs, 25 to 30 minutes. Bake times vary by oven. Start checking at 20-25 minutes. Transfer the pan to a wire rack and cool completely, about 1 hour and 30 minutes. Do not rush this step.

5. Zest and juice the remaining lemon, you need 1 teaspoon fresh lemon zest and 2 tablespoons fresh lemon juice. Whisk together with 1 cup of sifted confectioners’ sugar in a large bowl until smooth. Add the remaining 1/4 cup confectioners’ sugar, 1 tablespoon at a time to the glaze if needed, to make a tart lemon glaze thick enough to drizzle without running off the bars. Use an offset spatula or the back of a spoon to spread the glaze over the cooled blondies or drizzle glaze on the top in a cris cross pattern. Garnish with small lemon slices and cut into bars.