Help with chocolate chess tarts boiling over
I wanted to make a lower calorie version of chess pies, so I took my chess pie recipe and made some substitutions (low fat buttermilk instead of whole milk, cooked in silicone muffin cups without a crust, and a little less sugar). The first batch I made was with coconut, and they turned out beautifully. They were most and tasted surprisingly rich. Emboldened, I decided to make chocolate chess tarts the next time. Instead of coconut, I used unsweetened, dark chocolate baking cocoa. These were also moist and rich tasting. But there was a problem: the filling clearly bubbled over the tops of the muffin cups. The filling looked like lace along the sides of each tart, the tarts were not as deep, and there was lots of filling on the pan underneath the muffin cups. My housemate didn't mind. He ate the generous amounts of chocolate crust on the pan (he said they tasted like the crusty edges of brownies), but I would rather have the filling stay in the silicone muffin cup.
Does anyone have any idea why the chocolate batter would bubble over when the coconut batter didn't? And does anyone have any suggestions for preventing this from happening? Lower or raise the oven temp? I definitely want to make these again. Each tart, assuming all the batter stays in the muffin cup, comes in just under 75 calories.