CRUST:
Crush biscuits in food processor or bag with rolling pin.
Mix with remaining ingredients then press into bottom of lined springform pan.
Refrigerate 30min.
FILLING:
Mix agar and water in a small pan; set aside.
Beat mascarpone, yogurt, sugar, and vanilla until smooth.
Melt butter and chocolate; stir. When warm (but not hot), gradually beat into mascarpone mixture.
Melt agar over low heat and immediately pour (through strainer) into mascarpone mixture, beating all the while to avoid lumps.
Immediately pour filling into crust. Do not scrape sides with spatula!
Refrigerate at least 3hrs.