To prepare a rhubarb pineapple upside down cake recipe, first you need to prepare the cake batter. First, you'll need the following ingredients: 1 cup (240 ml) sugar, 2 tablespoons butter, 1 teaspoon vanilla, and 1 tablespoon of the flour. Mix the dry ingredients with the wet ingredients until they're just combined. Pour the batter into the prepared cake pan and bake it in the oven for 1 hour and 20 minutes or until the toothpick comes out clean. Let it cool for 10 minutes before removing it from the pan.