Trying to make a flourless chocolate cake. When adding the egg whites to the yolk/chocolate mixture, the chocolate clumps.

Egg whites need to be gently folded in to the choc/yolk mixture. Works best if done in batches. Take 1/3 of the egg whites and mix it in until just incorporated. Then gently fold in the remaining 2/3's. It helps to use a large spatula to do the folding.