1 cup butter
1 cup brown sugar
1 egg
1 teaspoon vanilla extract
1 1/2 cups white sugar
1 1/2 cups packed light brown sugar
3 tablespoons rum
1 cup miniature marshmallows
1 cup chocolate chips
Preheat oven to 350 degrees F (175 degrees C).
In a small saucepan, melt butter. Mix brown sugar, egg, vanilla, and white sugar; stir until well blended. Mix in rum, marshmallows, chocolate chips, and lemon zest. Pour mixture into a 9x13-inch pan.
Bake 35 to 40 minutes in the preheated oven, until a toothpick inserted into the center of the cake comes out clean.