How to Make It
Line a 9-by-13-inch baking dish with parchment paper; coat with nonstick cooking spray.
Whisk flour, salt, and baking soda in a medium bowl. Whisk brown sugar, oil, granulated sugar, and melted butter in a large bowl until combined. Add eggs, egg yolks, and vanilla; whisk until smooth. Stir in flour mixture just until combined. Stir in oats and mix-ins.
Press dough into prepared dish. Cut into quarters (cutting all the way through to parchment), then make shallow cuts, creating 12 squares in each quadrant (do not cut all the way through).
Cover dish with plastic wrap and freeze until dough is solid, about 1 hour. Unwrap, separate dough quadrants, and place in 4 freezer bags. Stack bags in freezer until ready to use.
To bake, heat oven to 350°F. Line a baking sheet with parchment paper. Break dough along scored cuts; place on sheet 2 inches apart. Bake, rotating halfway through, until edges are set and golden but centers are soft, 12 to 14 minutes (cookies will crisp as they cool). Let cool on wire racks.