3 tablespoons butter, cut into small cubes and chilled;
1 tablespoons heavy cream.
How to make
Boiled dressing
Sift the flour, mustard, sugar, salt, and white pepper into a small, heavy saucepan. Whisking constantly, add the cold water in a slow, steady stream, whisking until smooth. Whisk in the yolks and vinegar.
Cook over medium-low heat, stirring constantly with a heat-proof spatula, until the sauce is thick enough to coat the spatula, about 3 minutes. The sauce can thicken abruptly, so watch it carefully and stir well around the edge of the pan.
Remove the pan from the heat and whisk in the butter until it melts. Whisk in the cream.
Transfer the sauce into a small glass bowl or jar, let cool to room temperature, then cover and refrigerate until chilled. Store covered and refrigerated for up to 1 week.