Place the butter in a medium microwave-safe bowl. Break the chocolate into small pieces directly into the bowl. Microwave it in 20-second intervals, stirring between each bout of heat, until the chocolate is about 75% melted. Stir, allowing the residual heat in the bowl to melt the chocolate completely. (Alternatively, place the chocolate and butter in a heatproof bowl and place over a saucepan containing about 1 inch of barely simmering water. Stir with a wooden spoon until the chocolate is melted and the mixture is smooth.) Let the mixture cool for a few minutes, then whisk in the egg yolks one at a time, mixing until smooth after each addition. Set aside.