Preheat your oven to 350℉ and butter an 8x8" baking pan.
Cream together melted butter, granulated sugar, brown sugar, egg, vanilla extract, and salt. Stir until smooth.
In a separate bowl whisk together flour, cinnamon, nutmeg, baking powder and white vinegar. Stir into mixture until a thick, almost cookie-dough like batter forms.
Spread the batter into an even layer in your baking pan. Whisk together the sugar and cinnamon for the topping and sprinkle in an even layer over the top of your batter. Lightly press the sugar into the top with your fingers or a spatula.
Bake for 20-25 minutes, rotating halfway through. When fully cooked the edges should pull away slightly from the pan and a toothpick inserted into the center should have just a few crumbs.
Place on a wire rack to cool completely, 1-2 hours. Cut into 9 squares.