Shares of Krispy Kreme (DNUT +30.68%) surged more than 30% on Thursday after the doughnut maker boosted its full-year sales forecast.Image source: Getty Images.Strengthening profitabilityKrispy Kreme's net revenue declined 12.4% year over year to $328.8 million in its fiscal third quarter, which ended on Sept. 27.The decline reflects the doughnut shop operator's shift toward a less capital-intensive, franchise-based business model. Roughly a quarter of the chain's systemwide sales -- the total dollar value ...