I wouldn't add whipping cream to lighten up the pastry cream. The filling supports the top layer. The weight of the top layer of the cake will cause the filling to ooze out of cake.

This is my favourite recipe for Boston Cream Pie. The filling is delicious.

Boston Cream Pie

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1/3 cup butter
1 cup sugar
1 egg
1-1/2 cups sifted all-purpose flour
2-1/2 teaspoons baking powder
1/4 teaspoon salt
2/3 cup milk
1/2 teaspoon vanilla

Cream the butter and sugar until light and fluffly. Add the egg and
beat until creamy.

Sift the flour, baking powder, and salt together. Add alternately with
the combined milk and vanilla.

Turn the batter into a greased and floured 9-inch round layer cake pan.
Bake in a 350� oven for about 30 minutes, or until the cake springs
back when lightly touched in the center.

Cool on a rack for 5 minutes, then turn out and cool completely; Place
the cake on a serving plate and split horizontally into two layers with
a serrated knife. Carefully remove the top layer.

Fill with the chilled Cream Filling. Replace the top half and pour the
warm Chocolate Icing over the top, spreading only to the edges. Chill.
(When allowed to stand for several hours, the filling seeps into the
cake, making the cake and filling seem almost as one.)

Cream Filling

1/2 cup sugar
3 egg yolks
3 tablespoons flour
1 tablespoon butter
1/4 teaspoon salt
1/2 teaspoon vanilla
1 1/2 cups milk

Combine the sugar, flour, and salt in a saucepan. Gradually stir in the
milk. Cook over low heat, stirring until the mixture comes to a boil.
Boil, stirring for 1 minute. Beat the egg yolks slightly. Gradually
stir in about half the thickened sauce. Return to the heat and cook.
stirring 2 or 3 minutes longer to take away the flour taste. Remove
from the heat and blend in the butter. Strain into a bowl. Cool for a
few minutes, then blend in the vanilla. Cover and cool, then chill.

Chocolate Icing

2 squares of chocolate
1 cup sifted icing sugar
2 tablespoons butter
1 tablespoons hot water
1 teaspoon vanilla

Melt the chocolate and butter the microwave on medium low power. Stir
until smooth, and blend in the icing sugar and hot water. Stir until
smooth and slightly thickened (do not beat). Stir in the vanilla. Use
immediately.

When I was in high school I worked in an Italian pastry shop. One of the frosting choices was "Chantilly" which, as I recall, was buttercream mixed with whipped cream. It worked very well with certain cakes. Pretty sure it is not the classic definition of chantilly but it was quite popular.

Jo

Chantilly is lovely....but not in a Boston Cream pie. That eggy filling is an integral part of the whole.....doesn.t need whipped cream....but for maybe as an extra topping.