2Beat butter, brown sugar, white sugar, eggs, and vanilla extract together in a bowl using an electric mixer until light and fluffy. Stir flour, baking powder, cinnamon, salt, and nutmeg into creamed butter mixture until batter is just combined; fold in oats and 1 cup chocolate chips. Pour batter into a 9x12-inch baking pan; sprinkle with remaining 1/4 cup chocolate chips.
3Bake in the preheated oven until the top is dry and the edges have started to pull away from the sides of the pan, about 40 minutes.