Coat a 9x4x2 metal baking pan with nonstick spray (this will help the plastic wrap stay in place). Line the pan with plastic wrap, allowing the plastic to extend 2-3 beyond the sides of the pan.
Working quickly to prevent the ice cream from melting, stir the vanilla ice cream, bourbon or rum and nutmeg in a medium bowl to blend. Spoon evenly into bottom of prepared pan. Sprinkle with half of the almonds and half of the crushed espresso beans. Freeze first layer of terrine for 45 minutes. Remove from freezer and add latte ice cream layer. Sprnkle with remaining almonds and crushed espresso beans. Freeze for 45 minutes. Remove from freezer and spread chocolate layer. Cover and freeze until firm, at least 4 hours or overnight.
Meanwhile, wisk the cocoa powder and maple syrup in a small heavy saucepan over medium-low heat until the cocoa dissolves and the mixture thickens slightl, about 5 minutes. Wishk in milk. (The sauce can be made 1 day ahead. Cover and refrigerate. Reheat before using.).
To serve, unwrap the ice cream terrine and invert onto a serving platter. Cut into 12 slices and arrange each on a plate. Drizzle with sauce.