1. Make the crust: Mix graham crumbs, melted butter, and brown sugar. Press a spoonful into the bottom of each serving glass.
2. Prepare cheesecake filling: Beat cream cheese, powdered sugar, and vanilla until smooth. In a separate bowl, whip cream to stiff peaks. Fold into the cream cheese mixture until fluffy.
3. Make key lime layer: Blend sweetened condensed milk, key lime juice, zest, and softened cream cheese until smooth.
4. Layer the parfaits: In small jars or cups, layer graham crust, cheesecake filling, and key lime cream. Repeat the layers if desired.
5. Top it off: Pipe whipped cream or leftover cheesecake filling on top. Garnish with graham crumbs, lime zest, and lime slices.