1 1/2 cups all-purpose flour
1/2 teaspoon baking powder
1/8 teaspoon salt
1 cup unsalted butter, softened
2 cups packed light brown sugar
3 large eggs (at room temperature)
1 1/2 teaspoons vanilla extract
3/4 cup semisweet chocolate chips
1 ounce bittersweet chocolate, coarsely chopped
Position a rack in the center of the oven and preheat to 350 degrees F. Butter a 13 x 9-inch baking pan. Dust the pan with flour and tap out the excess.
In a medium bowl, using a wire whisk, stir together the flour, baking powder and salt. In a large bowl, using a hand-held mixer set at medium speed, beat the butter and brown sugar for 2 to 3 minutes, until the mixture is light in color and texture. Scrape down the sides of the bowl with a rubber spatula. Beat in the eggs one at a time, beating well after each addition. Beat in the vanilla extract.
At low speed, beat in the flour mixture in two additions, scraping down the sides of the bowl after each addition. Using a wooden spoon, stir in the chocolate chips. Scrape the batter into the prepared pan and smooth the top with the rubber spatula. Bake for 35 to 40 minutes or until a wooden pick inserted in the center comes out clean but not dry. Do not overbake. The chocolate chips will sink to the bottom of the blondies as they bake. Cool the blondies completely in the pan set on a wire rack.
Melt the chocolate. Dip a fork into the melted chocolate and shake and flick it over the top of the blondies to form a decorative zigzag pattern. Allow the chocolate to set for 15 to 20 minutes.
Using a large sharp knife, cut the blondies lengthwise into 4 strips, then cut each strip into 6 pieces to make 24 squares. The blondies can be stored, layered between sheets of wax paper, in an airtight container at room temperature for up to 3 days.