If you are making molten lava cake and the recipe calls for 6 squares of bitterseet chocolate and you only have unsweet chocolate do you need to increase the powdered sugar?

Yes, if you're substituting unsweetened chocolate for bittersweet chocolate, you should increase the powdered sugar in the recipe. Bittersweet chocolate contains sugar, while unsweetened chocolate does not, so adding extra sugar will balance the overall sweetness of the cake. A general guideline is to add about 1 tablespoon of sugar for each ounce of unsweetened chocolate used.