While my recipe calls for separating the egg and using the white to make a meringue before folding this into the batter, it isn't strictly necessary. You could instead make the cake batter in this recipe by keeping the 2/3 cup of sugar together (creaming it all together with the butter in step 4) and then adding the full egg to this. You would then carry on with the remainder of step 4, skip steps 5 and 6, and return to the recipe from step 7. This should still bake up OK, but may not raise as much and won't be as light and airy a finished product.