Cut the pound cake into 9 (3/4-inch) slices and spread each slice on 1 side with raspberry jam, using all the jam. Set aside.
Place a layer of cake, jam side up, in the bottom of a 2 1/2-to 3-quart glass serving bowl, cutting the pieces to fit. Sprinkle with Framboise. Top with a layer of raspberries and strawberries and Cognac Cream. Repeat the layers of cake sprinkled with Framboise, raspberries, strawberries and Cognac Cream, ending with a third layer of cake jam side down and raspberries and strawberries.
Whip the heavy cream in the bowl of an electric mixer fitted with the whisk attachment. When it starts to thicken, add the sugar and vanilla and continue to whip until it forms stiff peaks. Decorate the trifle with whipped cream. The trifle can sit for a while at room temperature.
Plain Pound Cake:
Preheat the oven to 350 degrees F. Grease and flour 2 (8 1/2 by 4 1/2 by 2 1/2-inch) loaf pans. Line the bottoms with parchment paper.
Cream the butter and granulated sugar in the bowl of an electric mixer fitted with the paddle attachment for about 5 minutes, or until light and fluffy. With the mixer on medium speed, beat in the eggs, 1 at a time.
In a large bowl, sift together the flour, baking powder, baking soda, and salt. In another bowl, combine the buttermilk and vanilla. Add the flour and buttermilk mixtures alternately to the batter, beginning and ending with the flour. Divide the batter evenly between the pans, smooth the tops, and bake for 45 minutes to 1 hour, until a cake tester comes out clean.
When the cakes are done, let them cool for 10 minutes. Take them out of the pans, place them on a baking rack and allow them to cool completely. Wrap well, and store in the refrigerator.
Heat the milk in a medium stainless-steel saucepan over medium heat and bring almost to a boil. Remove from the heat.
Beat the egg yolks and sugar on medium-high speed in the bowl of an electric mixer fitted with the paddle attachment until pale and thick, about 5 minutes. With the mixer on low speed, sprinkle on the cornstarch. Beat on medium-low speed until combined, scraping down the bowl with a rubber spatula.
With the mixer on low speed, slowly pour the hot milk mixture into the egg mixture. Pour the mixture back into the pan. Cook over low heat, stirring constantly, until the mixture thickens, 5 to 7 minutes. (Pay attention because it will thicken and then quickly become scrambled eggs!)
Immediately, pour the mixture through a fine sieve into a large bowl. Stir in the vanilla, Cognac, butter, and heavy cream. Place plastic wrap directly on the custard and refrigerate until cold.