Rhubarb Upside Down Cake

Name: Rhubarb Upside Down Cake
Prep time: 15 min
Cook time: 35 min
Total time: 50 min
Cuisine: American
Ingredients: 1 pound (455 g) rhubarb; 1/4 cup (57 g) unsalted butter, (cut into pieces); 3/4 cup (149 g) sugar; 1/2 cup (120 ml) lactose-free whole milk; 1 1/2 teaspoons lemon juice; 1 1/4 cups (181 g) plus 1 tablespoon low FODMAP gluten-free all-purpose flour,; 2 teaspoons baking powder; (use gluten-free if following a gluten-free diet); 1 teaspoon cinnamon; 1/4 teaspoon salt; 6 tablespoons (85 g) unsalted butter, (at room temperature); 3/4 cup (160 g) firmly packed light brown sugar; 1 teaspoon vanilla extract; 2 large eggs, (at room temperature)