Preheat your oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, leaving some overhang on the sides for easy removal.
In a large mixing bowl, beat the softened butter, brown sugar, and granulated sugar together until light and fluffy, about 3-5 minutes.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract until smooth and glossy.
In a separate bowl, sift together the all-purpose flour, baking powder, cream of tartar, salt, and cinnamon to evenly combine.
Gradually fold the dry ingredients into the wet mixture using a spatula. Mix just until combined to keep the blondies super soft.
Spread the batter evenly into the prepared pan. Combine 1/4 cup granulated sugar with about 1 teaspoon cinnamon and generously sprinkle this topping over the batter.
Bake for 25-30 minutes, until edges are golden and a toothpick inserted in the center comes out with a few moist crumbs. Avoid overbaking.
Allow to cool completely in the pan on a wire rack. Use the parchment paper overhang to lift out and slice into squares. Serve as is or warm with ice cream.