Mustard Spiced Chocolate Cookies are from a recipe that was sent to me years ago by a Colman's Mustard PR person. My old notes say "The cookies are good, but different. The best I can describe them is 'sharp'. They're chocolate cookies with an edge; not spicy or heat producing, just kind of, oh, I don't want to say 'metallic' because that's not complimentary, but the mustard boosts the bitterness in the chocolate so the flavor lingers on your tongue for a while".
I've made the cookies again since and my opinion is pretty much the same. Honestly, I don't think mustard belongs in dark chocolate cookies. Just because something exists doesn't mean it should be in cookies. Also, the recipe sent was designed for a UK audience as it calls for margarine and self-raising flour. USA margarine works, but most of the brands have been reformulated to have less fat, so you have to find one with at least 82% fat (Olivio, Country Crock, Earth Balance). As for the flour, self-rising isn't as popular here, so most people are probably going to see this and dump in too much AP which will make the cookies too dry. And then there' the mustard. It calls for a tablespoon. You can taste it. You could probably get by with a teaspoon, but I guess the point is to kind of taste it in conjunction with the orange and other flavors.
Anyhow, Colman's website is probably the best place to go if you're looking for ways to use dry mustard, but if you want to play around with this recipe here it is. One nice thing about it is it is egg-free, so if you are egg and dairy free you can use a vegan margarine and a vegan milk or coconut milk based yogurt in place of the sour cream. I recommend making a half batch first.
Mustard Spiced Chocolate Cookies
225g (8oz) soft margarine (16 tablespoons softened butter)
225g (8oz) caster sugar (1 cup plus 2 tablespoons plus 1 teaspoon sugar)
30ml (2 tbsp) natural yogurt or sour cream
30ml (2 tbsp) orange juice
15ml (1 tbsp) Colman's Dry Mustard
50g (2 oz) cocoa powder (unsweetened)
300g (11 oz) self raising flour (2 cups plus ⅓ cup plus 1 tablespoon flour)**
1-2 cups chocolate chips (optional)
Beat together butter, sugar, yogurt (or sour cream), orange juice and mustard. Sift (or just mix) cocoa powder and flour and add to the mix, stirring to a soft (mine was pretty stiff, actually) dough.
Form into smooth walnut-sized balls, arrange on a baking sheet and bake for 12 minutes**. Set oven at 375 deg. F, 190 deg. C or Gas mark 5.
Cool on a wire rack. Makes about 45 cookies.
**Original recipe said 15-20 minutes, but the cookies would have dried out.
** If you don't weigh your flour, fluff it up well before scooping and leveling smooth. If you use too much flour, the cookies will be dry.