Preheat and prepare the pan: Preheat oven to 350°F (175°C). Lightly butter a 9x13-inch (23x33 cm) baking pan and line with parchment if you prefer easier removal. Preheating ensures even rise; butter/parchment prevents sticking and gives clean cake pieces for a tidy trifle.

Mix and bake the cake: In a large bowl combine the chocolate cake mix, 3 large eggs, 1/2 cup vegetable oil and 1 cup water. Whisk just until combined — you want a smooth batter but overmixing incorporates excess air and can make the cake tough. Pour into prepared pan and bake 25–30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs (not raw batter). Baking to this point gives a tender, slightly moist crumb that will soak well but not become soggy in the trifle.

Cool and break the cake: Let the cake cool in the pan for 15 minutes, then invert onto a wire rack to cool completely (about another 15–20 minutes). Cooler cake is easier to handle; hot cake will melt the whipped cream and ganache, making layers runny. Once cool, cut or tear the cake into 1–1½-inch chunks — slightly irregular pieces look beautiful in a trifle and provide surface area for the sauces to cling to.

Macerate the strawberries: While the cake bakes, combine the sliced strawberries with 2 tablespoons granulated sugar and 1 tablespoon lemon juice in a bowl. Toss gently and let sit 15–30 minutes. Maceration pulls juices out of the berries, concentrating sweetness and creating a bright sauce that will keep the trifle vibrant and not purely cloying.

Prepare the chocolate ganache base: Place 1 cup (about 170 g) chocolate chips in a heatproof bowl. In a small saucepan, bring 1/2 cup (120 ml) of the heavy cream just to a simmer — small bubbles at the edge, do not boil. Pour the hot cream over the chips, let sit 60 seconds, then stir until smooth and glossy. The hot cream melts the chocolate evenly and creates a stable, shiny ganache that will carry intense chocolate flavor into the whipped cream.

Make chocolate chantilly (whipped chocolate cream): Chill a mixing bowl and beaters for 10 minutes in the fridge for better whipping. In the chilled bowl, whip the remaining 1/2 cup heavy cream with 2 tablespoons confectioners' sugar and 1/4 teaspoon salt to soft peaks (you should see billowy peaks that fold but still hold shape). Let the ganache cool to room temperature (so it is pourable but not hot) and gently fold roughly half of the ganache into the whipped cream to create a light, mousse-like chocolate chantilly. Folding gives volume; adding cooled ganache keeps the cream aerated while imparting rich chocolate taste. Reserve the other half of the ganache to drizzle between layers or warm slightly and use as a sauce.

Prepare the bananas: Slice the bananas just before assembly and toss gently with a teaspoon of lemon juice (from the maceration) to prevent browning and brighten flavor. Bananas brown quickly due to oxidation; a touch of acid keeps them fresh-looking and adds a subtle lift.

Assemble the trifle (individual glasses or a single bowl): Start with a layer of cake pieces covering the bottom (about 1/3 of the cake). Spoon a thin stream of reserved ganache over the cake — this helps the cake take on moisture and ties the layers together. Add half of the macerated strawberries (with a few spoonfuls of their juices) and a layer of banana slices. Dollop about one-third of the chocolate chantilly and spread gently. Repeat: another layer of cake, a drizzle of ganache, the remaining strawberries and bananas, and more chantilly. Finish with remaining chantilly, a scattering of the reserved 1/4 cup chocolate chips and an elegant drizzle of any leftover ganache. Layering provides texture contrast and ensures each spoonful has cake, fruit and cream.

Chill briefly and serve: Chill assembled trifle for at least 15 minutes to let flavors marry and the chantilly set slightly — 1 hour if you can. Serve slightly chilled; if served too cold, the chantilly will be firmer and flavors muted, if too warm the layers will sag. A short rest gives the ideal spoonable texture.

Finishing touches: Just before serving, add any fresh sliced strawberries for color and texture, and optionally grate a few chocolate chips over the top for a restaurant finish. Small fresh garnishes keep the presentation bright and signal the components inside.