In a small bowl, combine graham cracker crumbs, melted butter, and sugar. Mix until the texture resembles wet sand. 2. Divide the crumb mixture evenly among 4 ramekins (about 2 tablespoons each). Press firmly into an even layer on the bottom using the back of a spoon or a small glass. Place in the refrigerator to chill while you make the filling. 3. In a medium bowl, beat the softened cream cheese with a whisk until smooth. Add the Greek yogurt, lemon juice, lemon zest, sugar, and vanilla. Whisk until completely combined and creamy, about 1 minute. 4. Spoon the yogurt filling over the chilled crusts, dividing evenly. Smooth the tops. Return to the refrigerator for at least 2 hours, until set. 5. Meanwhile, make the lemon curd: In a small saucepan, whisk together the egg, sugar, lemon juice, and lemon zest. Cook over medium-low heat, whisking constantly, until the mixture thickens and coats the back of a spoon, about 4–5 minutes. Do not let it boil—if you see bubbles, reduce heat immediately. 6. Remove from heat and whisk in the cold butter cubes, one at a time, until smooth. Strain through a fine-mesh sieve into a small bowl to remove any cooked egg bits. Let cool to room temperature, then cover and refrigerate until needed. 7. Once the cups are fully chilled, spoon a thin layer of lemon curd over each (about 1 tablespoon per cup). Return to the fridge for another 30 minutes to set the curd. 8. Serve cold. Garnish with fresh mint or a pinch of lemon zest if desired.