It can be disappointing to cut into a well decorated Boston Cream Pie, only to have the filling run out over the plate because it's too thin. A Boston Cream Pie, really a cake rather than a pie, should have a thick, pudding-like filling sandwiched between two layers of sponge cake and topped with chocolate ganache. When the filling is too runny, it either soaks into the cake layers or runs. Most filling recipes call for cornstarch, and if you undercook or overcook the starch, you will end up with a thin filling.
Chill the runny cream pie filling. The filling needs to be cold to set up properly, and chilling it for several hours in the fridge will help you determine how much additional thickening it needs.
Sift into your filling bowl one tablespoon cornstarch for every cup of filling liquid. Whisk the cornstarch in thoroughly, until the filling is smooth and no starchy lumps or streaks remain.
Scrape the filling into your saucepan and heat over medium heat, stirring constantly to avoid scorching.
Bring the filling to a slow bowl, so that three to five large bubbles pop on the top of the filling. Your filling will have started thickening now.
Reduce the heat to low and simmer, without stirring, for 30 to 45 seconds, or until the filling is thick and glossy.
Scrape the filling into a clean bowl. Do not stir the filling after this point. Place a clean piece of cling film directly on top of the filling and chill the filling in the fridge for several hours, or until very thick.
Both undercooking and overcooking can cause runny filling. With undercooking, you are not breaking down the enzymes in the eggs that destroy the starch, nor are you cooking out the raw flavour of the starch or allowing it to thicken at the proper heat level. Overcooking, on the other hand, breaks down the cornstarch so that it can no longer thicken properly, resulting in a runny filling. Do not cook the filling at a boil or simmer for more than one minute.