Prepare your pan. Line a small baking sheet or rimmed pan with parchment paper.
Make sure it fits flat in your freezer.
Mix the yogurt base. In a bowl, stir together the yogurt, honey or maple syrup, and vanilla. Taste and adjust sweetness. The mix should be lightly sweet, not dessert-sweet.
Slice the apple thin. Core the apple and slice it very thin—think paper-thin semicircles or small matchsticks.
Toss slices with a little lemon juice to prevent browning and add brightness.
Spread the yogurt. Pour the yogurt onto the lined sheet and spread it into an even layer, about 1/4-inch thick. Thinner freezes faster and snaps cleanly.
Add toppings. Arrange apple slices evenly over the yogurt. Sprinkle cinnamon across the surface.
Add nuts or seeds, coconut, and a tiny pinch of flaky salt if using.
Freeze until solid. Slide the tray into the freezer and chill for 2–4 hours, or until completely firm. Overnight is fine.
Break into pieces. Lift the parchment out, then crack the bark into pieces. You can go rustic or cut it with a knife for clean squares.
Store it right. Keep the pieces in a freezer-safe container or zip-top bag with parchment between layers.
Freeze up to 1 month for best texture.