To the stainless steel bowl, add Dijon mustard, the egg yolks, lemon juice, and a generous pinch of salt. Vigorously whisk the mixture until the egg yolks turn a pale yellow color and thicken in consistency. Gradually pour in ½ cup of the warm butter, whisking continuously until the mixture becomes thick and creamy. Taste, and season with salt and cayenne powder. Remove bowl with hollandaise sauce and set aside.