In a bowl, blend the butter with the anchovy paste and refrigerate. In a small saucepan, boil the demiglace with the herbes de Provence until reduced to › cup, 3 minutes. Strain the sauce into a clean saucepan and stir in the vinegar. Bring the sauce to a simmer. Off the heat, whisk in the anchovy butter, 1 tablespoon at a time. Season with salt and pepper, cover and keep warm.