Preheat oven to 350°F; rub an 8 or 9 inch cast iron skillet with oil.
Beat butter at medium speed with an electric mixer until smooth and creamy. Add 1 1/4 cups sugar, and mix until fluffy.
Add eggs and vanilla; mix well.
Add flour, 1/2 tsp cinnamon, salt, and baking powder. Beat until blended.
Add white chocolate chips and mix well.
Spread dough in the prepared skillet. Stir together 1 tbsp sugar and 1 tsp cinnamon; sprinkle evenly over cookie dough. (You don't have to use all of it if it seems like a lot. I just really love cinnamon. :P )
Bake 28 to 30 minutes or until golden brown and center is set.
Let cookie cool slightly. (It's best served a little warm!)