In a mixing bowl, whisk together egg yolks, cornstarch, ¼ cup milk, and salt. Set aside.
In a large pot, mix together 2¾ cup milk and sugar. Bring to a simmer over medium heat.
Once mixture comes to a simmer, turn off heat and pour half into the egg yolk mixture and whisk to combine well and then pour into the pot with remaining milk and sugar mixture. Stir in lime zest and lime extract until well combined.
Stir well until custard thickens.
Spoon custard into mini pie crust and spread evenly.
Place a piece of plastic wrap or parchment paper on top of custard, pressing down lightly to cover the entire pie filling.
Place in the fridge to cool and set up for 2 hours.
In a large bowl, using an electric hand mixer, beat heavy whipping cream, sugar, and vanilla until stiff peaks form.
Pipe whipped cream on top of the chilled custard. Top each pie with lime zest and a lime slice, if desired.