Put the nut butter and 1 tablespoon of powdered sugar into a large mixing bowl with 1 tablespoon of heavy cream. Mix until smooth, then add more cream, 1 tablespoon at a time, until the mixture is thin enough to add the rest of the cream without becoming lumpy. Whisk to soft peaks then transfer to a piping bag fitted with a fluted nozzle.
Slice the doughnuts in half across the middle and pipe rosettes of praline cream onto the bottom layers. Top with the doughnut lids then dust generously with powdered sugar.