In a small bowl mix together the flour, sugar and mustard powder until evenly blended. Whisk together the milk and egg yolks in a saucepan and slowly whisk in the mustard mixture. Stir in the vinegar. Whisking constantly, bring the mixture to a boil over medium heat, reduce heat to low and simmer until thickened.
I followed this recipe a little. I used 3Tbsp sugar and 1 large Tbsp cornflour mixed them together in a bowl. I had no powder so when following wet ingredient recipe of two yolks and 1 cup of milk I added a good amount of American mustard. I did add more once the dry and wet ingredients were thickening until it tasted good. I didn't add the vinegar instead using a mix of boiled water from cabbage and leeks and carrots as the sauce co tongued to thicken - 01 Oct 2016