• Combine melted butter with brown sugar, salt, and vanilla, mixing with a rubber spatula until combined.
• Let the mixture cool slightly if necessary, so it won't scramble the eggs. Then add the eggs and stir well to combine. You want them mixed in, but you don't need to beat air into them.
• Stir in flour and baking powder, mixing until all the dry ingredients are moistened. Add chocolate chips.
• Spread batter in a greased 9x13-inch pan, all the way to the edges. The batter will be thick.
• Bake at 350F for about 30 minutes. The top will become golden brown and shiny all over, and the edges will look done. A toothpick inserted into the middle in a spot without chocolate should come out a bit damp or wet, for gooey bars. If there's thick batter on the toothpick, bake a couple of minutes longer.
• Cool completely before cutting and removing from the pan.

  I often cut a row after 15 minutes, so that I can eat them immediately. They are still fragile and fall apart, so I recommend only cutting them when you want to eat them.
• Store in an airtight container at room temperature. Chocolate chip cookie bars are moist and delicious for a few days. If you feel they've dried out a little, reheat a cookie bar in the microwave for 5 seconds.

Less Gooey Chocolate Chip Cookie Bars

• Soften the butter, instead of melting it. Use an electric mixer, and cream the butter with the brown sugar, salt, and vanilla until it's fluffy and lightened, about 3 minutes.
• Add the eggs one at a time, mixing for 30 seconds after each addition. Use the mixer for the remaining steps as well.
• Bake the cookie bars for an extra 2-3 minutes. Stick a toothpick into the center, in a spot without chocolate. It should come out with moist, dense crumbs, but not batter. The toothpick will not be clean.