Shortbread with Cumberland Rum Butter

1/2 cup unsalted butter (at room temperature)
1 1/4 cups all-purpose flour, sifted
Superfine sugar (for dusting)
1 cup unsalted butter
2 1/4 cups dark brown sugar
1/4 teaspoon grated nutmeg
1/3 cup dark Jamaican rum

Preheat oven to 300 degrees F.

In a large bowl beat the butter with a wooden spoon until it is soft, then continue beating while adding sugar then the flour. When it gets hard to handle, bring it together with the spoon and make into a ball with your hands. Transfer to a working surface that has been lightly dusted with superfine sugar. Quickly and lightly roll out about 1/8 inch thick. You may need to dust the rolling pin with sugar, also.  With a 3-inch diameter crinkled cookie cutter, cut out the cookies and place on a cookie sheet. Bake on a high shelf in the oven for about 30 minutes, until they are a pale golden color ? no darker. Cool the cookies on a wire rack and dust them with some superfine sugar. When cool, store in an airtight container.

Melt the butter in a small heavy-bottom saucepan over low heat. As butter is melting, put the sugar in a large mixing bowl and remove all lumps by pressing them out with your hands or a wooden spoon. Add nutmeg. Pour on the rum and mix well. Pour on the melted butter and stir in well. Put mixture into a glass bowl, cover the surface with wax paper and, when cool, place in the refrigerator until serving time.