Softened butter and powdered sugar well. Alternately, one by one add the egg yolks (be at room temperature and the eggs are fresh), the mixture constantly breaks. Add the rum and stir gently - kernels. For the cream milk mixed with rum. Biscuits one by one is a quick dip in milk and piecing a row in a baking dish with removable bottom. Place a third of the cream. Place a new layer biscotti, again 1/3 cream and biscotti third layer. Finally, a last third cream and smoothed. Put it in the refrigerator to harden the cream. Then remove the side of the baking dish and sprinkle with cocoa or decorate otherwise. Serving to stay in the refrigerator.