Mix: Combine 1 box of Betty Crocker Brownie Mix with the water, oil, and eggs in a medium bowl until well blended.
Sift & Cream
Sift together the All-Purpose flour, salt, and baking soda.
In a stand mixer with a paddle attachment, cream the 125 g of butter and sugars at moderate speed for 5-8 minutes until light and fluffy.
Emulsify & Incorporate
Combine eggs and vanilla; add to the mixer in stages, scraping the bowl thoroughly after each.
Add the dry ingredients on low speed in thirds. Fold in the chocolate chips and walnuts manually.
Sheet Pan Assembly
Line one half-size sheet pan with parchment paper. Spray corners and sides of pan AND the parchment lightly with pan spray.
Spread the prepared brownie batter equally into the pan. Spread it to within 1/2-inch of the edges.
Use a #24 scoop to portion cookie balls, flatten each into a 1/2 inch disc, then position them evenly over the brownie batter in a 5 x 5 grid (25 total).
The Convection Bake
Preheat convection oven to 325°F.
Bake for 22-28 minutes.
Note: If the cookies reach a deep golden brown before the 22-minute mark, loosely tent with foil for the final 5 minutes.
Finishing & Yield
The brookies are done when the cookie layer is golden brown and the brownie base jiggles slightly in the center when the pan is moved.
Allow to cool completely on a wire rack in the pan. For the cleanest cuts, chill the pan briefly in the reach-in.