PREHEAT oven to 300F (150C). In a medium bowl, using an electric mixer or wooden spoon, beat butter until creamy. Gradually beat in sugar and rum flavouring. Continue beating until well mixed, about 2 min. In a small bowl, using a fork, stir salt and 1 tsp (5 mL) nutmeg into flour until mixed. Using a wooden spoon, beat into the butter mixture just until combined.
BUTTER or spray a 9 × 13-in. (3-L) glass baking dish. Turn dough into dish. With lightly floured fingers, evenly press dough into dish. Using a fork, gently prick dough at ½-in. (1-cm) intervals. Bake in centre of preheated oven just until top is golden, 50 to 55 min. Remove dish to a rack and gently prick again. Cut into squares while still warm. Cool completely in pan. For topping, stir 2 tbsp (30 mL) icing sugar with 1/2 tsp (2 mL) nutmeg. Sift over cooled baked bars. Store in an airtight container in a cool place up to 1 week or freeze up to 1 month.