Preheat oven to 350 degrees F. Line a 9x13 baking dish with parchment paper, leaving some overhang for easy removal.
Place butter in a large microwave safe mixing bowl and microwave on high for a minute, and then in 20 seconds intervals (if necessary) until melted, making sure it doesn’t boil. Add in shortening, both sugars, eggs, and vanilla. Beat with a handheld mixer until combined. Add in flour, baking powder, cream of tartar, and salt. Continue to mix until well blended. (Batter will be thick.)
Using a spatula, transfer batter to the prepared dish and spread it out evenly. Mix together the 1/8 cup of sugar and cinnamon; sprinkle it over the top.
Bake for 30 minutes until a toothpick inserted in the center comes out almost clean.
Remove from the oven and allow to cook completely before separating from the parchment paper. Cut into bars and enjoy!
These can be stored for up to four days at room temperature in an airtight container.