Preheat oven to 325°F (160°C). Line an 8×8-inch square baking pan with parchment paper, leaving overhang for easy removal.

1. In a bowl, combine graham cracker crumbs, sugar, and melted butter. Press firmly into the bottom of the pan. Bake for 10 minutes. Let cool slightly.
2. In a large bowl, beat cream cheese until smooth. Add Greek yogurt, sugar, lemon juice, zest, and vanilla; mix until combined.
3. Beat in eggs one at a time. Add cornstarch and mix until just incorporated.
4. Pour cheesecake batter over crust and smooth the top.
5. Bake for 30–35 minutes, or until center is set but still slightly jiggly. Turn off oven, crack the door, and let bars cool inside for 30 minutes.
6. Chill bars in the refrigerator for at least 4 hours or overnight.
7. To make the meringue, beat egg whites and cream of tartar until soft peaks form. Gradually add sugar, beating until stiff, glossy peaks form. Add vanilla if using.
8. Spread or pipe meringue over chilled cheesecake layer.
9. Toast meringue with a kitchen torch or under the broiler until golden. Chill briefly before slicing and serving.