This recipe calls for brown and yellow mustard seed; however, it also requires some mustard powder. I would think that you can grind enough mustard seed to give you 1 cup of powder, using it as directed. Maybe a mix of brown and yellow depending on how sharp you prefer the mustard.
INGREDIENTS: 1 cup brown mustard seeds (brown seeds will produce sharp mustard), 1⁄4 cup yellow mustard seeds (yellow produces mild mustard), 1 cup dark beer, 1 1⁄4 cups white wine vinegar, 1 cup mustard powder, combined with 1 cup water (let sit 20 minutes), 1 teaspoon sugar. 1 teaspoon salt. 1 teaspoon allspice, ground 1⁄4 teaspoon turmeric, ground. 1⁄4 teaspoon mace, ground.
In a non-reactive container, combine mustard seeds with beer and vinegar. Let sit 48 hours. Check periodically to make sure seeds are covered by liquid; add more if necessary.
Transfer seeds and liquid to a food processor. Add remaining ingredients. Process until creamy, four to six minutes.
Transfer to jars. Allow mustard to mature, 3 to 4 weeks, before serving.
Note: You can vary the mustards flavor by substituting your favorite ingredients, the basic technique remains the same.