Beat egg yolks with remaining 1/4 cup sugar, until very light. Add olive oil to yolks, and beat well. Add liqueur and cooled chocolate to the yolks. Fold yolk mixture into the egg whites, by hand, until well combined.
Freeze in individual mini-cups or in 9-inch round springform pan.
TIP - NO MESS WAY TO TRANSFER MOUSSE TO MINI-CUPS: Use a large Ziploc bag, and carefully add chocolate mousse to bag. Zip top of bag shut, cut a small piece off one bottom corner of bag, and squeeze the mousse out through the small hole at the bottom of the bag.