Preheat your oven to 325°F (165°C). Line a standard 12-cup muffin tin with cupcake liners.
In a medium bowl, combine graham cracker crumbs, granulated sugar, and salt. Pour in melted butter and mix until the mixture resembles wet sand.
Place approximately 2 tablespoons of crust mixture into each cupcake liner. Use the bottom of a small glass or measuring cup to firmly press the crumbs into an even layer with a slight rim up the sides.
Bake the crusts for 5 minutes, then remove from oven and allow to cool slightly while preparing the filling.
In a large bowl, whisk egg yolks until slightly thickened. Add condensed milk and whisk until smooth.
Gradually add lime juice while whisking constantly. The mixture will begin to thicken naturally. Fold in lime zest and cream of tartar if using.
Carefully pour the lime filling into each pre-baked crust, filling each about ¾ full. Bake for 10-12 minutes, until the centers are just set but still have a slight jiggle.
Allow the pies to cool at room temperature for 30 minutes, then refrigerate for at least 2 hours or ideally overnight.
Just before serving, whip cold heavy cream until soft peaks form. Add powdered sugar and vanilla extract, then continue whipping until stiff peaks form.
Pipe or spoon whipped cream onto each chilled mini pie. Garnish with lime zest or thin lime slices.