The texture of the batter for this pumpkin cake should be substantial enough to support the chocolate chips while baking. However, if you are baking in a hot kitchen or baking a cake with a thinner batter, you might have had this issue. You can try tossing the chocolate chips in a little flour before adding them to the batter or substitute mini chocolate chips which are lighter and tend not to sink as much as regular. I also like to add scatter a handful of chocolate chips over the top of the cake before baking for a nice effect.