Preheat the oven to 450°F and line a large baking sheet with parchment paper. Lightly flour a work surface and roll the pizza dough in an 8"x9"x1" rectangle. Cut the dough into 30 squares (6x5).
Take each square of dough and wrap a piece of mozzarella inside, pinching to seal into a ball.
Arrange the balls, seam side down, on the baking sheet, in a Christmas tree shape working upwards, with 2 balls for the trunk, then 7 balls, 6, 5, 4, 3, 2, and 1 for the top of the tree.
Whisk the egg with 1 tbsp water. Brush the tops of the balls with egg was and bake the tree until golden and cooked through, about 15-20 minutes.
While the tree is baking, make the garlicky butter. Melt the butter over low heat and stir in the garlic, Parmesan, and basil.
When the tree is ready, remove from the oven and brush generously with the garlic butter. If desired, finish with more Parmesan, basil, and red pepper flakes, if desired.
Enjoy warm!