Mix butter, brown sugar and melt on low heat. Add the crushed biscuits, remove from fire and mix well. Lay this in a tart tray. Flatten it with the back of a spoon till even. Let this cool in the refrigerator for 20mins.
Melt the cooking chocolate on a double boiler or in microwave in short bursts. When slightly cool, add the cream ad beat at high speed till well combined. Add the lemon juice, rind and mascarpone - beat well.
In a separate bowl. Beat egg whites, powdered sugar and vanilla till fluffy and they form a stiff peak.
Gently fold this into the chocolate mixture till well combined. Pour this into the biscuit base and let it refrigerate for 2hrs.