Preheat your oven to 350 degrees Fahrenheit and line a jelly roll pan with foil. Place 2 mini tart molds on the pan.
Slice the tops off of 2 grapefruits (slice the top ¼ off).
Using a spoon, swirl inside the grapefruit to separate the segments from the pith. Scoop the fillings into a bowl (snack on it while you make the cake!). Use a spoon to get any other portions of the filling out (be careful not to scrape off the pith).
Place the empty grapefruits into the mini tart molds so that they stand upright.
In a small bowl, whisk together the coconut flour, erythritol, baking powder and salt.
In a large bowl, whisk together the applesauce, milk, yogurt, eggs, zest and extracts. Whisk vigorously until the entire mixture is even.
Dump the dry ingredients over the wet ingredients and whisk vigorously so that there are no clumps. When the mixture is even, carefully pour the batter into the grapefruit molds. It should fill up about half of the grapefruits, but don't worry, it'll rise to the top as it bakes.
Bake for ~65 minutes, or until the surface of the cakes spring back when tapped. Let cool completely. Slice the grapefruits in half and serve!