Preheat the oven to 350°F (175°C). Prepare a 9x13-inch baking pan by lightly greasing it and lining the bottom with parchment. Preheating ensures even rising and consistent crumb structure; lining the pan makes it easy to remove the cake without breaking it.

Make the cake batter: in a large bowl combine the chocolate cake mix, 3 large eggs, 1/2 cup vegetable oil and 1 cup water. Whisk until smooth and no dry streaks remain. Mixing until just combined gives the cake a tender crumb — overmixing will develop gluten and make it dense.

Pour the batter into the prepared pan and smooth the top. Bake 25–30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Baking to 'moist crumb' rather than bone-dry keeps the cake soft for layering; it will finish setting as it cools.

Remove cake from the oven and allow it to cool in the pan on a wire rack for 15 minutes; then invert onto the rack and cool completely. Cooling on a rack prevents excess steam from making the bottom soggy and helps you cut neat cubes.

While the cake cools, macerate the strawberries: hull and slice the strawberries, place them in a bowl with 1 tablespoon granulated sugar and 1/4 teaspoon lemon juice (or a pinch of salt if you prefer). Gently toss and let sit 10–15 minutes. Macerating draws out juices and concentrates sweetness so the berries sing against the rich chocolate.

Prepare the chocolate chantilly (whipped chocolate cream): chill a metal mixing bowl and the whisk for 10 minutes in the fridge (cold tools help cream whip faster). Pour 1 1/2 cups of the heavy whipping cream into the chilled bowl with 1/4 cup powdered sugar and 1 teaspoon vanilla. Whip on medium-high until soft peaks form. Cooling the bowl minimizes temperature rise so the cream holds structure.

Make a quick chocolate ganache to deepen the chocolate flavor: heat 1/2 cup (about 85 g) of heavy cream in a small saucepan until it just begins to simmer (bubbles at the edge) — do not boil. Remove from heat and pour over 1 cup (170 g) semisweet chocolate chips in a heatproof bowl. Let sit 1 minute to melt, then whisk until glossy and smooth. Stir in 2 tablespoons butter for shine and silkiness. Warming the cream and letting the chips sit melts chocolate evenly without seizing; butter enriches and improves mouthfeel.

Slice the cooled cake into 1-inch cubes. Cubes are ideal for layering because they create even layers and soak up just enough of the ganache and fruit juices for flavor without disintegrating.

Caramelize the bananas (optional but recommended): slice the bananas on the diagonal into 1/3-inch slices. Heat a nonstick or stainless pan over medium heat with 1 teaspoon butter. Add banana slices and cook 30–60 seconds per side until golden — do this quickly so they hold their shape. Caramelizing adds a toasty, deeper flavor that complements chocolate and the fresh berries.

Assemble the trifle in a large glass bowl or 8 individual glasses: start with a layer of cake cubes (about 1/3 of the total), drizzle 3–4 tablespoons of warm ganache over the cubes (warm ganache will sink slightly, adding moisture and chocolate flavor), add a layer of macerated strawberries and a few caramelized banana slices, then a generous layer of chocolate chantilly. Repeat 1–2 more times, finishing with chantilly on top. Alternating textures and temperatures gives every spoonful contrast — chocolate, fruit, cream and a touch of crunch.

Garnish: sprinkle the remaining 1/2 cup (85 g) chocolate chips on top (or roughly chop and toast them briefly for crunch). For extra gloss, drizzle any leftover warm ganache. A final sprinkle of flaky sea salt brightens the chocolate.

Chill the assembled trifle 20–30 minutes before serving to let flavors meld and the chantilly set slightly. If you prefer a firmer set, chill up to 2 hours. Chilling makes cleaner slices and allows the ganache to settle into the layers.

Serve chilled or at cool room temperature. Use a deep spoon to scoop through all layers so each serving has cake, fruit, and cream. The combination of warm ganache and cool chantilly is particularly lovely if you serve immediately after assembling.