To make the crust, preheat the oven to 350°F. Process the graham crackers into fine crumbs in a food processor, then add the melted butter, granulated sugar, and salt and process until all the crumbs are moistened. Press evenly into the bottoms of four 4-inch tartlet pans with removable bottoms, or one 8-inch tart pan with a removable bottom, and all the way up the side of the pan(s), packing it tightly with your fingertips so it is even and compacted. Bake until crisp, 5 to 7 minutes. Let cool completely on a wire rack. (The crusts can be wrapped in plastic and frozen for up to 1 month.)
To make filling, combine the butter, brown sugar, and corn syrup in a medium-size heavy saucepan and bring to a boil, stirring frequently. Stir in the cream, walnuts, espresso powder, and salt and return to a boil. Reduce the heat to medium-low and cook at a simmer (not a boil), stirring occasionally, for 12 minutes.
Scrape the hot filling into the prepared tartlet shells. Let cool to room temperature. Cover with plastic wrap and refrigerate until completely set, at least 3 hours and up to 1 day, before serving.
Excerpted from Icebox Desserts, by Lauren Chattman. (c) 2005, used by permission from The Harvard Common Press.
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