BBQ Peel and Eat Shrimp with Lemon Mayo

Name: BBQ Peel and Eat Shrimp with Lemon Mayo
Ingredients: 2 cups all-purpose flour; 1 3/4 teaspoon baking powder; 1/2 teaspoon fine-grain sea salt; 4 ounces (1 stick) unsalted butter, room temperature; 1/2 cup white granulated sugar; 2 large eggs; 1 teaspoon pure vanilla extract; 3/4 cups whole milk; 6 tablespoons unsalted butter; 3/4 cup brown sugar; 1 rosemary sprig, leaves detached; 1 large honey crisp apple, cored and thinly sliced (equaled to about 1 cup of sliced apples); Pinch of flaked sea salt, plus more topping