Preheat the oven to 350°F. Spray an 8×8-inch baking pan with nonstick spray.
In a medium bowl, whisk together applesauce, soy milk, maple syrup, vanilla extract, eggs, and melted coconut oil until smooth.
Stir in rolled oats, protein powder, baking powder, cinnamon, allspice, and salt. Mix until fully combined.
Gently stir in the finely diced Honeycrisp apple.
Pour mixture into the prepared baking dish and smooth the top evenly. Arrange thin apple slices on top.
Bake for 35–40 minutes, until the edges are lightly golden and the center is set.
In a small bowl, whisk together Greek yogurt, maple syrup, soy milk, and cinnamon until smooth.
Let bars cool slightly, top with glaze if using, slice into squares, and serve.