Buff Orpington carry two of the genes needed to produce autosomal barring ( as in campine pencilling) they are Co*Co (columbian) and Db*Db (dark brown). Buff Orpington are usually wheaten at the E locus but can be wild type also. It appears that your Pitta carry Pg*Pg (pattern gene) and also carry birchen at the E locus. Many self-black fowl carry the pattern gene because it is a eumelanizer (adds black to feathers). The offspring from the cross only carry half the genes needed to express good autosomal barring. The chocolate down in the chicks is due the expression of the dark brown gene on a birchen ground color.
Birds that are purebred for birchen, dark brown and the pattern gene produce autosomal barring. Some autosomal barred breeds also carry the columbian gene plus the previously mentioned genes.
If you could produce a male and a few females cross them and pick the best autosomal barred F1 offspring and then cross them together. Keep crossing the best offspring and in a few generations you should have some nice looking birds.
Here she is a bit older and yes Id say penciling for sure. Really confused about the leg color but maybe the spotted legs over the white legs does something like this.