• Adjust an oven rack to the middle position; preheat oven to 325 degrees. Spray a 8×8-inch baking pan with nonstick spray. Line baking pan with parchment paper or foil and spray with nonstick spray again.

For the blondies:

• In a medium bowl whisk the flour, salt, and baking powder together until combined. Set aside.
• Whisk the melted butter and brown sugar together in a medium bowl until well combined. Add the egg and vanilla and mix well. Using a rubber spatula, fold the dry ingredients into the egg mixture until just combined. Do not overmix. Fold in the chocolate chips and turn the batter onto the prepared pan, smoothing the top with a rubber spatula. Set aside.

For the brownies:

• In a medium bowl, whisk the flour, salt, and baking powder together until combined. Set aside.
• Melt the chocolate and butter in a large heatproof bowl over a saucepan of almost-simmering water, stirring occasionally, until smooth. Alternatively, in a microwave, heat the butter and chocolate in a microwave-safe bowl on high for 45 seconds, then stir and heat for 30 seconds more. Stir again, and, if necessary, repeat in 15-second increments until melted. When the chocolate mixture is completely smooth, remove the bowl from the saucepan and gradually whisk in the sugar. Add the eggs, one at a time, whisking after each addition until combined. Whisk in the vanilla. Add the flour mixture in 3 additions, folding with a rubber spatula until the batter is smooth.
• Spread brownie batter evenly over blondie batter. Bake 30-35 minutes* (see note below). Cool on a wire rack to room temperature, then remove the blondie-brownies from the pan. Cut into 2-inch squares and serve. Blondie-brownies can be stored in an airtight container at room temperature for up to 3 days.
• *Note: Sometimes I like a bit of gooey, undercooked goodness in my brownies but if you prefer a firmer texture go ahead an bake the full 35 minutes, possibly longer. You can determine how cooked your bars are using a cake test or skewer and judging from the amount of crumbs that stick.