1. Preheat oven to 400°F. Line two baking sheets with parchment paper.
2. Combine flour, cream of tartar, baking soda, salt, and 1/4 teaspoon nutmeg in medium bowl. With electric mixer, beat 1 cup butter and 1 1 /2 cups sugar on medium high until light and fluffy, about 2 minutes. Add eggs and vanilla and mix until incorporated. Reduce speed to low, add flour mixture, and mix until just combined.
3. Place remaining 1/2 cup sugar and 1/2 teaspoon nutmeg in a small bowl and stir to combine. Roll dough into 72 (1-inch) balls and roll in sugar mixture. Place balls 2 inches apart on prepared baking sheets. Bake both trays until edges are lightly browned, 8 to 10 minutes, switching racks and rotating sheets halfway through baking. Transfer to wire rack to cool. Repeat with remaining dough.
4. Whisk remaining 4 tablespoons softened butter, rum, cream, and salt together in medium bowl. Whisk in confectioners’ sugar until smooth. Turn half of cooled cookies over (bottom side up) and spread each with 1 teaspoon of the filling. Sandwich with another cookie. 

Tip: If you prefer not to use rum in the filling, you can substitute an additional 2 tablespoons cream for the rum.