How to Make It
Combine first 3 ingredients in top of a double boiler. Cook over medium-high heat until reduced to 2 tablespoons; add 1/2 cup hot water. Whisk in egg yolks. Add about one-third of butter to egg mixture; cook over hot, not boiling, water, whisking constantly, until butter melts. Add another third of butter, stirring constantly. As sauce thickens, stir in remaining butter. Cook until temperature reaches 160°, stirring constantly. Stir in rosemary. Remove sauce from double boiler; serve immediately.