Filling: In the bowl of a stand mixer (or in a medium bowl, using an electric hand mixer), beat mascarpone cheese on High for about 1 minute, until creamy. Add condensed milk and lime juice; beat on Medium for about 1 minute, until smooth. Gently fold in whipped topping, until blended. Place 12 silicone muffin liners in a 12-cup muffin tin. Scoop mixture into cups until three-quarters full.
Crust: In a medium bowl, combine graham cracker crumbs, melted butter and cardamom, stirring well. Sprinkle crust mixture evenly over each mini pie. Press gently with the back of a spoon to flatten. Freeze for about 2 hours, until firm.
Make It Now: Remove from freezer. Gently remove silicone liners and invert mini pies onto a serving plate, crust side down. Top with additional whipped topping and a slice of Key lime, if using. Serve frozen.
Make It a Freezer Meal: Remove from freezer. Transfer mini pies, in their silicone liners, to 2 labeled gallon-size (4 L) freezer bags. Seal, removing as much air as possible, and freeze.
Serve: Remove from freezer. Gently remove silicone liners and invert mini pies onto a serving plate, crust side down. Top with additional whipped topping and a slice of Key lime, if using. Serve frozen.