I’ve dealt with runny chocolate cream pies before. A runny pie needs thickening. Check the filling recipe. Did you use enough cornstarch or flour? These thickeners are crucial.
Re-cook the filling. Pour the filling back into a saucepan. Heat it over medium heat. Whisk in a slurry. Mix equal parts cornstarch and cold water, about 1 tablespoon each. Stir until the filling thickens.
Cool the filling properly. Let it set in the fridge for several hours. This helps it firm up.
Use gelatin for extra stability. Dissolve 1 teaspoon of gelatin in 2 tablespoons of cold water. Add it to the warm filling. This will help set the pie.
Chill the pie thoroughly. Allow it to sit overnight if possible. A well-chilled pie holds its shape better.
Prevent future issues. Ensure you cook the filling until it thickens before pouring it into the crust. Always let it cool completely before serving.
For more detailed pie-making tips, check out King Arthur Baking and Bon Appétit.
Good luck with your pie! I’m sure it will turn out great.