Place all the ingredients, except the
butter and pepper, in a bowl and whisk until frothy. Set the bowl over a pan of gently
simmering water and continue to whisk until the whisk leaves a
trail. Remove from the heat and whisk in the
butter a little at a time, beating after each addition, until
thickened. The sauce should be frothy and of a
pouring consistency. Taste and adjust the seasoning and
serve at once.