Preheat the oven to 425°F / 220°C.
Place the pizza crust on a baking sheet or pizza pan.
Spread the apple pie filling evenly over the crust, leaving a small border around the edges.
Sprinkle cinnamon over the apple filling, if using.
In a bowl, combine the flour, butter, sugar, vanilla extract, and salt.
Mix with a fork or your fingertips until the mixture becomes crumbly.
Sprinkle the streusel topping evenly over the apple filling.
Bake for 15 minutes, or until the topping is lightly golden and the apple filling is warm and bubbly.
In a small bowl, whisk together powdered sugar, milk or water, and vanilla until smooth.
Drizzle the glaze over the warm pizza.
Slice and enjoy while warm.