Preheat oven to 350°F. Lightly butter an 8-inch square baking pan with a teaspoon of butter then line with parchment paper.
Combine 1/2 cup melted butter, eggs and vanilla extract in a large bowl.
Add the contents of the jar and stir to combine.
Transfer batter to prepared baking pan and bake until golden brown and a toothpick inserted into the center comes out clean, about 25-30 minutes.