Preheat the oven to 350°F (177°C). Line a 9x13-inch baking pan with aluminum foil, making sure to leave some overhang for easy removal, and set aside.
In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until well combined. Add the eggs and vanilla and mix until fully combined.
Add the flour, baking powder, cream of tartar, and salt to the wet ingredients and mix until just combined.
Scoop the dough into the pan and spread it out evenly.
In a separate bowl, mix together the granulated sugar and cinnamon. Sprinkle it evenly over the cookie dough in the pan.
Bake for 25 to 30 minutes or until the tops of the bars are set.
Remove from the oven and allow to cool in the pan. Cut into slices and enjoy.
Notes
Storage instructions: Store blondies in an airtight container at room temperature for up to five days.Freezing instructions: Blondies may be frozen for up to two months, just thaw in the refrigerator overnight.For a thicker cookie bar: These can be baked in an 11x7-inch pan instead. Adjust baking time to 30-35 minutes at 350°F.