Preheat your oven to 350°F (175°C) and grease a 9x13 inch pan.

Combine the cake mix, eggs, melted butter, and vanilla extract in a large mixing bowl. Stir until well blended.

Fold in your choice of add-ins until evenly distributed.

Pour batter into the prepared baking pan and smooth the top with a spatula.

Bake for 20-25 minutes, until the edges are golden brown and a toothpick comes out with moist crumbs.

Cool in the pan for 10 minutes before transferring to a wire rack to cool completely. Slice into squares.