Heat oven to 350°F. Line an 8-inch square pan with parchment, leaving overhang.
Stir together graham crumbs, sugar, and salt in a bowl. Mix in melted butter until evenly moistened.
Press crust firmly into the pan. Bake 8–10 minutes until fragrant. Cool 5 minutes.
Whisk condensed milk, egg yolks, key lime juice, lime zest, and sour cream until smooth and creamy.
Pour filling over the warm crust and tap the pan to release bubbles.
Bake 14–16 minutes, until edges are set and center has a slight jiggle.
Cool to room temperature, then chill at least 2 hours until firm.
Whip cream with powdered sugar and vanilla to soft peaks. Spread or pipe over chilled bars.
Lift out with parchment. Slice into bars, wiping the knife between cuts.