The breed is D'uccle - porcelain is just a variety (color/pattern type) of d'uccle, as is mille fleur. You can cross the two colors and still have a "pure" d'uccle. Its kind of like breeding a black poodle to a white poodle - they're still "pure" poodles, just different colors.
Genetically, porcelain is just visual lavender added to mille fleur.
You can breed a mille fleur d'uccle and a porcelain d'uccle together - you would get visual mille fleur offspring that is split to (carries) lavender in the F1 generation.
In order to get porcelain in the F2 generation, you would need to breed the F1's to a porcelain (ie. possibly their porcelain parent), or to another mille fleur split to lavender (ie. possibly breed F1 siblings together).