Preheat your oven to 350 degrees F. Grease a 9x13 inch baking pan with butter or nonstick spray, then line it with parchment paper leaving a little overhang on the long sides so you can lift the bars out cleanly.

Brown the butter first. Melt it in a medium saucepan over medium heat, stirring occasionally. Keep cooking after it melts, about 4 to 5 minutes, until the butter turns a golden amber color and smells nutty. Small brown specks will form on the bottom of the pan. Pull it off the heat right away and pour it into your large mixing bowl so it stops cooking.

Whisk the brown sugar into the hot browned butter until fully combined. Let it cool for 5 minutes so it does not scramble the eggs, then whisk in both eggs and the vanilla extract until the mixture looks smooth and slightly glossy.

Stir in the sweet potato puree with a rubber spatula until no streaks remain. Add the flour, cinnamon, nutmeg, ginger, baking powder, and salt. Fold gently until just combined. A few small lumps are fine. Do not overmix or the blondies will be tough.

Spread the batter evenly into the prepared pan. Bake for 20 to 22 minutes until the top looks set and a toothpick inserted in the center comes out with moist crumbs but no wet batter.

Scatter the mini marshmallows evenly across the top of the blondies. Return the pan to the oven and bake for another 8 to 10 minutes until the marshmallows are puffed and golden brown on top. Watch them closely in the last few minutes as they go from golden to burned quickly.

Remove from the oven and let the pan cool completely on a wire rack, at least 1 hour. The marshmallow layer needs to set before you cut. Use the parchment overhang to lift the slab onto a cutting board, then slice into 24 bars with a sharp knife. Wipe the blade between cuts to keep the marshmallow from dragging.