Preheat oven to 350°F (175°C). Grease a 9-inch square cake pan. Pour melted butter into the pan and sprinkle evenly with brown sugar.
Arrange pineapple slices in a single layer over the sugar mixture. Scatter cranberries evenly in the gaps.
In a medium bowl, whisk together flour, baking powder, and salt.
Cream softened butter and granulated sugar until light and fluffy. Add eggs one at a time, then mix in vanilla.
Add dry ingredients alternating with milk, beginning and ending with dry ingredients. Mix just until combined.
Pour batter evenly over the fruit layer. Bake for 40–45 minutes, until a toothpick inserted in the center comes out clean.
Cool in the pan for 10 minutes, then invert onto a serving plate. Cool slightly before slicing.