Let butter soften to room temperature. Spray a 9x13 inch pan with nonstick cooking spray. Preheat your oven to 325°.
Cream butter and brown sugar in a large bowl for 3 minutes. (I use a hand mixer, but you can use a Kitchenaid.) Add the eggs and vanilla and beat till smooth.
In a small bowl, whisk together the flour, baking powder, cream of tartar, salt, cinnamon, and nutmeg. Stir into the dry ingredients with the cinnamon chips. Use a light touch to prevent tough bars.
Press dough into prepared pan. Combine topping ingredients and sprinkle over the top of the dough.
Bake at 325° for about 35-40 minutes, or just until the middle is barely set. For moist bars, don't overbake!
Cool for at least 30 minutes and cut into squares.
Notes
I have tried these with both cinnamon and butterscotch chips. Both varieties are fabulous!
I’ve learned that they will not appear to be done in the middle, but will set up as they cool. Don’t overbake or they turn hard.