How to Make It
Preheat oven to 350°.
Combine 2 tablespoons butter and brown sugar in a small saucepan over medium heat, stirring until melted and smooth. Spread brown sugar mixture into bottom of a 9-inch square metal baking pan coated with cooking spray and lined with parchment paper.
Drain pineapple, reserving 1/4 cup juice. Top brown sugar mixture with pineapple slices.
Weigh or lightly spoon flours into dry measuring cups; level with a knife. Combine flours, baking powder, and xanthan gum, stirring with a whisk. Combine milk and reserved 1/4 cup pineapple juice in a small bowl.
Place 4 tablespoons butter and granulated sugar in a large bowl; beat with a mixer at medium speed until light and fluffy. Add vanilla and eggs; beat until blended. Add flour mixture and milk mixture alternately to sugar mixture, beginning and ending with flour mixture, stirring until combined.
Pour batter evenly over fruit. Bake at 350° for 40 minutes or until a wooden pick inserted in center comes out clean. Immediately invert cake onto a serving platter. Cool 1 hour.