Preheat your oven to 350°F (175°C). Grease an 8x8-inch baking pan and set aside.
In a medium bowl, whisk together melted butter and sugar until smooth. Add eggs and vanilla extract, mixing until fully combined.
Sift in cocoa powder, flour, salt, and baking powder. Gently fold the ingredients together until just combined.
Pour the batter into the prepared pan and spread evenly. Bake for 20-25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Allow the brownies to cool, then cut them into squares.
Assemble the sundae
Place a warm brownie on each plate or bowl.
Top each brownie with a scoop of vanilla ice cream.
Drizzle chocolate syrup or hot fudge sauce over the ice cream and brownie.
Add whipped cream, a sprinkle of chopped nuts, and a maraschino cherry if desired.