Preheat oven to 350°F. Line a 9x13 baking pan with aluminum foil, making sure to leave some overhang for easy removal, and set aside.
In a large bowl, mix together the melted butter, brown sugar, and granulated sugar until well combined. Add in 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.
Spoon the batter into the pan and spread it out evenly.
In a separate bowl, mix together the granulated sugar and cinnamon. Sprinkle it evenly over the mixture in the pan.
Bake at 350°F for 25-30 minutes.
Remove from the oven and allow to cool in the pan. Cut into slices and enjoy.
Store blondies in an airtight container at room temperature for up to five days.
Blondies may be frozen for up to two months, just thaw in the refrigerator overnight.
For a thicker blondie, these can be baked in an 11x7 pan. Adjust baking time to 30-35 minutes at 350°F.