Preheat oven to 375°F and set a rack in the middle of the oven. Lightly butter a 9-inch tart pan with removable bottom.

Combine pecans, cocoa powder, flour, granulated sugar and ¼ teaspoon salt in a food processor fitted with a steel blade. Pulse several times, until the mixture becomes a coarse meal. With the processor running, slowly pour the melted butter through the feed tube. Process until all ingredients are moistened. Add an additional teaspoon of melted butter if ingredients fail to stick together when pinched between your fingers.

Press pecan mixture firmly and evenly into the bottom and up the sides of the prepared tart pan. Use a straight-sided, round measuring cup to further compact and evenly smooth out the bottom and sides. Chill crust in the freezer for 15 minutes. Once chilled, place tart shell on a baking sheet and use a fork to poke several holes over the bottom of the tart to prevent it from bubbling up. Bake until firm, 12 to 15 minutes. Allow crust to cool while you make the filling.

Reduce oven temperature to 350°F. In a medium bowl, whisk together pumpkin purée, condensed milk, maple syrup, egg, egg yolks, vanilla, cinnamon, ginger, nutmeg, allspice and remaining ¼ teaspoon salt. Pour filling into the cooled crust. Place the filled tart on a baking sheet and bake until filling is loosely set and the top begins to brown in spots, 45 to 50 minutes (custard will continue to set as it cools). Remove tart from the oven and cool to room temperature on a wire rack. When cool enough to handle, carefully remove the outer ring of the tart pan and chill the tart in the refrigerator for at least 1 hour.

When ready to serve, sprinkle the top of the tart evenly with the turbinado sugar. Use a kitchen torch and move the flame evenly back and forth just above top of tart, avoiding crust, until sugar is caramelized and bronzed. Let sugar cool until hardened, about 5 minutes. Serve immediately as the crackling caramel will begin to soften and eventually melt as it absorbs moisture from the filling.