Slice the zucchini into 1/2-inch rounds. Arrange on a paper towel-lined sheet pan in a single layer and sprinkle lightly with kosher salt. Let rest for 10-15 minutes to draw out excess moisture, then pat thoroughly dry with paper towels.

Transfer dried zucchini rounds to a bowl and toss with 2 tablespoons olive oil, black pepper, and garlic powder until evenly coated.

Preheat an outdoor grill or grill pan over medium-high heat until very hot. Lightly brush the grates or pan surface with olive oil.

Add the zucchini rounds in a single layer — do not crowd them. Grill undisturbed for 2-3 minutes per side until distinct grill marks form and the zucchini is just tender but still holds its shape. Work in batches if needed.

Transfer grilled zucchini to a clean plate and let rest for 1-2 minutes.

On a serving platter or individual plates, place a grilled zucchini round as the base. Top with a slice of fresh mozzarella, then a slice of tomato, then a basil leaf. Repeat with a second zucchini round, another slice of mozzarella, tomato, and basil to create a two-layer stack. Repeat to make approximately 8 stacks total.

Drizzle the balsamic glaze generously over the assembled stacks, then finish with a drizzle of extra-virgin olive oil.

Sprinkle with flaky sea salt and red pepper flakes if using. Garnish with additional fresh basil leaves. Serve immediately.