Preheat oven to 350°. Line a 13x9-inch baking pan with foil, letting foil hang over sides of pan. Spray with cooking spray.
In a large bowl, beat together butter, brown sugar, and granulated sugar with a mixer at high speed until light and fluffy, approximately 3 minutes. Scrape down sides of bowl. Add eggs, one at a time, beating well after each addition. Beat in vanilla extract.
In a medium bowl, whisk together flour, baking powder, and salt. Add to butter mixture, beating to combine. Beat in chocolate chips and walnuts at low speed. Spread batter into prepared pan, smoothing with an offset spatula.
Bake until blondie edges are light golden brown and a wooden pick inserted in center comes out clean, approximately 20 minutes. (Don’t overbake, or blondie will be too firm.) Let cool completely in pan on a wire rack.
Using excess foil as handles, lift blondie from pan, remove foil, and place on a cutting surface.
Melt chocolate melting wafers according to package directions. Transfer melted chocolate to a piping bag with a small hole cut in tip. Drizzle chocolate in diagonal lines over cooled blondie and let chocolate harden.
Using a long, sharp knife and pressing downward for clean cuts, trim and discard rough edges from blondie and cut into 48 bars, dipping knife in hot water and wiping dry between cuts. Store blondies in an airtight container.
Notes
Kitchen Tip: Measure flour by spooning it into a dry-measure cup and then scraping off excess with a knife to level.
Recipe by TeaTime Magazine at https://teatimemagazine.com/chocolate-chip-walnut-blondies/