Start by coloring the apple slices. Whisk water with 1/2 cup sugar in a small pot over medium-high heat until sugar is dissolved (about 1-2 minutes).
Then, add hibiscus flower and let mixture come to a boil. When liquid turns deep red, remove the spent hibiscus flowers using a slotted spoon or spider strainer and stir in lemon juice.
Place the peeled apple slices into the pot and let cool to room temperature. Transfer everything to a pitcher or clean airtight container, cover, and refrigerate overnight (or at least for 1-2 hours). The longer the apples steep, the redder the color!
Alternatively, you can use jamaica agua fresca powder to color the apples instead of the hibiscus flower. For this, whisk water with jamaica agua fresca powder (skip sugar and lemon) in a small pot over medium-high heat (about 1 minute) and let mixture come to a boil. Remove pot from heat and add apple slices, letting sit until liquid cools to room temperature.
Then, transfer everything to a pitcher or clean airtight container, cover, and refrigerate overnight. Again, the longer it steeps, the redder the color!
Right before preparing the cake batter, pour all the contents of the pitcher into a colander and let apples drain free of the liquid for about 5-10 minutes.
Meanwhile, preheat oven to 350°F. Spray a 9-inch cast-iron skillet with baking spray with flour, or alternatively grease with unsalted butter and flour the skillet. Set aside!
Prepare the cake batter. In the bowl of a stand mixer fitted with the paddle attachment, beat softened unsalted butter and remaining 1 1/2 cups sugar at medium speed for 3-4 minutes or until fluffy. You may have to stop mixer to scrape down the sides of the bowl.
Add eggs, one at a time, beating well after each addition. In a separate bowl, whisk together flour and salt.
Gradually add flour mixture to butter mixture alternately with 1/3 cup milk; begin and end with flour mixture, beating just until combined after each addition. Then, mix 1 1/2 teaspoons almond extract and spoon batter into reserved skillet.
Place colored apple sliced cut side down in a pinwheel fashion over batter. Bake on middle rack for 25 to 30 minutes or until lightly browned.
Sprinkle with sliced almonds, and bake on lower rack (to prevent burning the nuts) for an additional 10-15 minutes or until a toothpick inserted in center comes out clean. Transfer skillet to a rack and let cake cool for about 10 minutes.
WHILE cake is cooling, prepare the glaze. Whisk together 2-3 tablespoons milk, remaining 1/2 teaspoon almond extract, and confectioners’ sugar until mixture is smooth and pourable (without lumps).
Drizzle over cake and let set. Serve cake warm with a good cup of coffee or tea. Enjoy!