Preheat oven to 425°; line a baking sheet with parchment paper.

For the shortcakes, whisk together flour, sugar, baking powder, and ½ tsp. salt in a bowl.

Cut 4 Tbsp. butter into flour mixture using a pastry blender until butter is the size of peas. Stir together ½ cup cream and yogurt in a second bowl, add to flour mixture, and blend with a fork just until dough comes together.

Pat out dough on a lightly floured surface into a circle about 1-inch thick. Using a 2½-inch round cutter, cut out as many shortcakes as possible and place on prepared baking sheet. Gently press scraps into a 1-inch-thick circle and cut remaining biscuits.

Brush shortcakes with cream and sprinkle with sanding sugar. Bake until golden, about 15 minutes.

For the toppings, simmer brown sugar, 1 stick butter, rum, liqueur, cinnamon, and pinch of salt in a large sauté pan over medium heat. Cook until thickened, about 8 minutes, whisking occasionally.

Add bananas and simmer until warmed though, about 1 minute. Off heat, stir in pineapple.

Halve shortcakes horizontally; coat cut sides with Foster sauce. Place bottoms on serving dishes, cut side up, then top with fruit mixture, ice cream, nuts, and remaining shortcake halves.