Sorry for my silence on the subject, I didn't notice this discussion. I think good suggestions have already been made.
Also, as I found out today, for the microwave version, making the mix too water-wet (my egg was small and I only had yoghurt to hand) made mine not rise much and stay quite cakey.
You can also try using baking SODA instead of baking powder- it reacts with the acidity in the FromFrais/Yoghurt. You need much less though, and it gives it a bit of a 'Soda Bread' taste...
And yes - adjust the cooking time, my MW is only 800W and 3 minutes seems to be exactly right for my container shape too.
As for spreading - yes I frequently have quark for the first moisture layer (instead of butter or philly, sometmes I just sprinkle the quark with fresh chives and a sprinkling of Salt&Pepper) but after that treat it like a normal sandwich: occasional lean ham, roasted sliced chicken/turkey, bresaola, tuna 'mayonnaise' (tuna, quark, chives, vinegar/lemon to taste), egg 'mayonnaise' (mashed up egg again with fromfrais and mustard), slices of smoked salmon, tinned salmon 'pate' with dill (made just as the two 'mayo' variations above). Or toast and cut into finger for dipping in soft-boiled egg.
Sweet things: Sweetened & Vanilla flavoured Quark / FromFrais, topped with SF/FF Jelly 'Jam' (I do that with the 'muffins' after splitting them horizontally). Or toast and dipped in sweetened / flavoured Yoghurt, I like that for breakfast too.
Think normal 'sandwich' and the possibilities are endless really. I usually also add some rocket / lettuce / sliced cucumber etc etc, and recently in Conso have also been adding sliced fruit for the sweet version.