Cooking does not affect lactose. And regarding the amount it depends how much lactase activity is left. If it’s none, even tiny amounts will cause problems. Heck, lactose intolerance prevented the settling of Vikings in North America! (Well - that’s probably not entirely true. The Natives one of the Viking boats encountered at one place were very friendly at first – and turned into fierce warriors driving away the Vikings after they were given a cheese as present. But facts shouldn’t stand in the way of a good story.)