Remove the pumpkin stem and cut in half; remove seeds. Cut each half into 3 pieces. Place in a steamer basket set over simmering water; cover and steam for about 15 minutes or until very tender. Cool, then scoop out flesh and mash; measure ½ cup of pumpkin.

In a mixing bowl, cream butter and brown sugar together. Add egg yolk and mix well, then mix in pumpkin. Add cinnamon, lemon zest and salt; blend well. Add flour and mix well. Transfer dough to a work surface. Divide in half and roll each half into a log about 1½ inches in diameter. Wrap in plastic and refrigerate until firm, at least 2 hours or overnight.

Preheat oven to 350°F; line a baking sheet with parchment. In a small bowl, whisk the egg white to blend. Brush dough with egg white, then roll in sugar to coat. Slice into rounds about ½ inch thick. Transfer cookies to baking sheet. Bake for about 20 minutes, until golden brown.