Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking pan or line it with parchment paper.
In a large mixing bowl, whisk together the melted butter and brown sugar until well combined.
Add the eggs and vanilla extract, whisking until the mixture is smooth and glossy.
In a separate bowl, whisk together the flour, baking powder, salt, cinnamon, and nutmeg.
Gradually add the dry ingredients to the wet ingredients, stirring with a spatula or wooden spoon until just combined. Be careful not to overmix.
If using, fold in the white chocolate chips for extra sweetness.
Pour the batter into the prepared baking pan and smooth out the top with a spatula.
Bake for 25-28 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with a few moist crumbs.
Allow the blondies to cool completely in the pan before slicing into squares.
Serve and enjoy!