1. Stir together first 2 ingredients in top of a double boiler. Cover and let stand 12 to 24 hours.
2. Pour water to depth of 1 inch into bottom of a double boiler over medium-high heat; bring to a boil. Reduce heat to low, and simmer; place top of double boiler over simmering water. Whisk sugar and eggs into mustard mixture, and cook, whisking constantly, 8 to 10 minutes or until thickened. Remove from heat, and cool completely (about 1 hour). (Mixture will continue to thicken as it cools.) Refrigerate in an airtight container up to 2 weeks.
Note: We tested with Colman's Mustard Powder.