This buttercream variation is exceptionally silky smooth and rich. It is very important that the pastry cream and butter be roughly the same temperature before beating them together.

Prepare and let cool to room temperature:

Have ready:

In a large bowl, or in a stand mixer with the paddle attachment, beat the pastry cream until smooth. With the mixer running on medium speed, beat in the butter slowly, a couple tablespoons at a time, beating after each addition until fully incorporated and smooth before adding the next. It should take about 3 minutes to incorporate all the butter. Beat until silky smooth. Use right away, cover and refrigerate for up to 5 days, or freeze for up to 3 months. Before using, let it come to room temperature, then beat well until smooth.