In a large bowl, cream together butter, brown sugar and sugar. Add vanilla and eggs and combine. Fold in flour, baking soda and salt.
Mix in 2 cups of mini chocolate chips.
Spray a 15 x 11-inch jelly roll pan with nonstick cooking spray and spread cookie dough out evenly in pan. Sprinkle remaining 1/4 cup chocolate chips on top.
Bake for 20-25 minutes or until middle is set up and edges are golden brown. Let cool and cut into squares.
Notes
If you use a smaller pan your batter will be a lot thicker, and needs more cooking time so it cooks all the way through.