Beat 1 cup (250 g) of the butter with the powdered sugar into cream in a container suitable for the freezer.
To the cream, add the yolks and coffee and stir with a wooden spoon until homogeneous.
Put the mixture in the freezer to harden. Take it out and form round balls; it's best to dip your fingers in cold water, to smooth out the bonbons nicely.
Melt the chocolate in a water bath with the rest of the butter. Pour this glaze over the bonbons.
Put them in the freezer briefly. Before serving, sprinkle the bonbons with powdered sugar.