Combine 3/4 cup sugar, flour and salt in top of a double boiler; add milk, stirring well. Cook over boiling water, stirring constantly, 20 minutes or until thickened. Beat egg yolks until thick and lemon colored. Gradually stir about 1/4 hot mixture into yolks; add to remaining hot mixture. Cook 15 minutes over boiling water, stirring frequently. Remove from heat; stir in butter and bourbon. Cool.