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Preheat the oven to 350°F with a rack positioned in the center. Line an 8" square pan with a parchment sling.
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To make the filling: In a large saucepot, heat the cream, sugar, salt, and lime zest over high heat until boiling, stirring occasionally with a heat-proof spatula to dissolve the sugar. Once the mixture comes to a boil, reduce the heat slightly to maintain a gentle boil for 5 minutes, stirring constantly. Watch the mixture carefully and further reduce the heat if the cream starts boiling rapidly — maintain a gentle boil. The mixture will thicken only slightly; it will look almost as fluid as plain cream.
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Remove the saucepot from the heat and stir in the lime juice. Set aside to cool slightly in the saucepot while you prepare the crust. Stir occasionally to prevent a skin from forming.
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To make the crust: In a medium bowl, stir together the graham cracker crumbs, confectioners’ sugar, and salt. Add the melted butter and mix until thoroughly combined.
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Transfer the crust mixture to the prepared pan, then use the bottom of a measuring cup or glass and the pads of your fingers to press it firmly and evenly into the bottom of the pan.
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Bake the crust for 10 to 15 minutes; it’ll darken in color a bit. Remove it from the oven and place it on a rack to cool for 10 minutes.
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Gently pour the filling over the slightly cooled crust, then carefully place the pan, uncovered, in the refrigerator for at least 6 hours, or overnight, until set.
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Once set, lift the bars out of the pan using the parchment sling and transfer them to a cutting board. For the cleanest and most precise cuts, soak a chef’s knife in hot water, dry it thoroughly, then slice the bars. Repeat soaking and drying the knife between cuts, as needed, to keep the edges clean.
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To top the Key lime bars: When ready to serve, top each bar with a dollop of whipped cream, then zest the lime over the top. These bars are best served chilled.
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Storage information: Store any leftover Key lime bars, covered, in the refrigerator for up to 1 week; freeze for longer storage.