In a medium saucepan over medium-low heat, melt ½ cup of unsalted butter, stirring frequently to prevent burning.

While the butter cools, grease an 8-inch square baking dish and preheat your oven to 350°F (175°C).

In a medium mixing bowl, combine 1 cup of light brown sugar, ½ cup of all-purpose flour, 1 teaspoon of baking powder, and ⅓ cup of unsweetened cocoa powder.

Add ¼ cup of milk and 1 large egg to the bowl with the slightly cooled browned butter and whisk until fully combined.

Gently fold the dry ingredient mixture into the wet mixture until just combined, then fold in 1 cup of evenly chopped rhubarb.

Pour the brownie batter into the prepared baking dish and bake for 20-25 minutes.