Preheat oven to 350 degrees. Grease a 9x13 baking pan.
Combine melted butter and brown sugar in a bowl.
Add eggs, vanilla, flour and nuts, if using. Mix well and pour into 9x13 baking pan.
Bake at 350 degrees for 10 minutes. Reduce temperature to 325 degrees and bake for an additional 22-25 minutes. Cool completely.