Preheat oven to 350 degrees F. Grease a cookie sheet or 8 x 8 inch cake pan and set aside.
To make the raspberry liqueur sauce, add all the sauce ingredients to a small saucepan. Cook over medium heat until mixture starts to boil (approximately 1 minute). Make sure to stir continuously throughout the process. Once the mixture starts boiling and mixture is smooth, remove from heat immediately.
To prepare the apples, slice off the tops of the apples, and with a spoon gut out the center of the apples, making them into mini bowls. Make sure to leave approximately 1 centimeter thickness for the apple walls, and not to puncture the bottom. Set aside.
To prepare the cheese filling, add the ricotta cheese, fontinella cheese, sugar, egg yolk, and raspberry liqueur to a stand mixer. Mix on low until fully combined. It should have a bit of a lumpy consistency. Separately, whip the egg white until soft peaks form, then add the whipped egg whites to the cheese filling. Mix gently using a spatula until fully combined.
To assemble the apples, spoon the cheese filling into the gutted apples. Leave a little space at the top of the apple and place the filled apples directly on the cookie sheet or cake pan and bake for approximately 20-25 minutes. Once slightly cooled, carefully plate the apples, top with crumbled biscotti and raspberry liqueur sauce.