Place the top of a double boiler or a heatproof bowl over a pan of hot water on low heat. Place yolks, lemon juice, salt, and pepper in a blender with 1/4 cup water. Puree until smooth. While machine is running, slowly drizzle in hot butter. Use immediately, or transfer to double boiler and keep warm over very low heat, whisking occasionally. Do not allow mixture to get too hot, or it may curdle.