Preheat oven to 350°F and line pan with parchment.
Combine melted butter and sugar; add eggs, molasses, and vanilla.
Whisk dry ingredients separately, then fold into wet mixture.
Spread in pan, bake 28–33 minutes until set.
Whisk milk and flour on medium heat until thick; cool.
Beat butter and brown sugar; add cooled mixture and spices.
Spread frosting over cooled brownies, slice, and serve.
