Preheat your oven to 350°F (175°C) and line a 9×9-inch baking dish with parchment paper.

In a large bowl, whisk together melted unsalted butter and light brown sugar until smooth. Add eggs, molasses, and vanilla extract, stirring until creamy.

In a separate bowl, whisk together all-purpose flour, ground ginger, cinnamon, baking powder, and kosher salt.

Gradually add the dry mixture to the wet ingredients, mixing until smooth without overmixing.

Pour the brownie batter into the prepared dish and bake for 28–33 minutes until edges are set.

Let the brownies cool completely in the pan on a wire rack for about 30 minutes.

In a saucepan, combine milk and flour over medium heat, whisking until thickened, about 3–5 minutes.

In a stand mixer, beat room temperature unsalted butter and light brown sugar until fluffy, about 3–4 minutes. Then add the cooled milk mixture and gingerbread spice.

Spread the gingerbread ermine frosting over the cooled brownies, slice into squares, and serve.