Preheat oven to 375°F. Line 2 baking sheets with parchment paper. In a food processor, process prunes and water until puréed. In a large bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, ginger and nutmeg. In a stand mixer fitted with a whisk attachment, beat together the prune purée, butter, and brown sugar. Add the egg and beat until the batter is slightly frothy, about 1 minute. Beat in the molasses. With the mixer on low speed, add the dry ingredients in three installments. Refrigerate the cookie dough for at least 1 hour or overnight. The dough might expand while it chills. Using an oiled ice-cream scooper, form the cookie dough into 25 gram / 1 1/2 Tbsp balls of cookie dough. Roll the top of the balls in the demerara sugar and then place on the prepared baking sheets, spacing the cookies about 2 inches apart. Press the balls to slightly flatten. Bake the cookies in the center of the oven until the tops are crinkled and edges are set, 12 to 15 minutes. Cool completely on wire racks.