First, you need an excellent, preferably home-made sponge cake, though you could use a store-bought Sara Lee pound cake easily enough. Slice the cake into 1/4-inch slices.
Make a quickie berry jam:
2 lbs frozen strawberries
1 lb frozen raspberries
1/4 cup water
1 cup sugar
2 tsp unflavored gelatin, hydrated in 2 Tbp water.
Bring to a simmer, and cook gently until thickened a bit. Mash the berries to make more homogeneous if necessary. Add gelatin. Let cool.
This will not set up like a “Jello” dessert, but the gelatin will give it a bit more body. It will not be as sweet as a commercial jarred jam.
Feel free to use any jarred jam you like, though I am particular to strawberry, raspberry, or apricot for trifles. I would NOT use Concord grape jam.
Make a recipe of pastry cream. (There’s really no good substitute for this. Cooked pudding comes close, but lacks the richness, as there are no eggs.)
1 qt milk
8 egg yolks
1 1/4 cup sugar
4 Tbp flour
4 Tbp corn starch
2 tsp vanilla extract
2 Tbp softened butter
Scald milk gently over moderate heat. (You can also do this in the microwave.) Set aside.
Beat egg yolks with sugar until smooth. Add flour and corn starch. Beat until smooth. Add about a 1/2 cup of the warm milk by dribbles into egg/sugar/flour mixture and beat well until the egg mixture has been warmed up a bit (this is called “tempering”), then pour the egg/milk mixture back into the remaining hot milk.
Cook over moderate heat, stirring constantly, until thickened like pudding. Remove from heat. Add vanilla extract. Beat in butter.
Cover surface with plastic wrap so that a skin does not form. Let cool completely before using.
Assemble your trifle in a straight-sided trifle bowl.
Layer sliced cake in the bottom. Generously dribble rum (or other alcohol like brandy). Spread cake with jam. Spread some of the pastry cream. Repeat several times, finishing with pastry cream.
Decorate with whipped cream on top, if desired.