Start to finish: 7 minutes, plus approximately 20 to 25 minutes baking time and 20 minutes cooling time
2 sticks unsalted butter (plus more for greasing the pan)
1 pound (2 cups firmly packed) light brown sugar
2 teaspoons baking powder
1/4 teaspoon salt
1 teaspoon pure vanilla extract
2 cups all-purpose flour (see Cook's note)
2 large eggs
1 cup semisweet chocolate chips
1 cup chopped pecans, optional
Preheat the oven to 350 degrees. Grease a 9-inch-by-12-inch baking pan well with butter. Set the pan aside.
Melt the 2 sticks of butter in a 2-quart microwave-safe bowl covered with a paper towel. Remove the bowl from the microwave, and add the brown sugar. Stir well. Add the baking powder, salt and vanilla extract, stirring until completely incorporated. Stir in the flour, again stirring until it is well mixed. The batter will be very stiff.
Crack the eggs into a small bowl, and beat them well with a fork or whisk. Add the eggs to the batter and stir with a spoon until they are completely incorporated. Stir in the chocolate chips (and the nuts, if using).