Preparation Steps

• Make the strawberry filling first: In a small saucepan, combine strawberries, sugar, lemon juice, jam, and a pinch of salt. Let stand 10 minutes to draw out juices, then set over medium heat and cook, stirring, until the berries are juicy and just starting to break down, 6–8 minutes. Stir the cornstarch with 2 teaspoons cold water, add to the pan, and simmer 1–2 minutes until thick and glossy. Scrape into a shallow bowl and chill completely.
• Prepare pans and oven: Heat the oven to 340°F (170°C). Grease three 8-inch round cake pans, line bottoms with parchment, and lightly dust the sides with cocoa powder. If you have cake strips, soak and wrap the pans for flatter layers.
• Bloom the cocoa: In a heatproof cup, whisk the cocoa and espresso powder into the hot coffee until smooth; set aside to cool slightly while you mix the batter.
• Mix the cake batter: In a large bowl, whisk the flour, sugar, baking powder, baking soda, and salt. In a separate bowl, whisk the eggs, vanilla, sour cream, and oil until smooth. Whisk the cocoa-coffee mixture into the wet ingredients. Pour the wet mixture over the dry and whisk just until evenly combined and glossy. Let the batter rest 5 minutes, then give it a final stir to bring up any settled bits.
• Bake: Divide batter evenly among the pans, tap each pan firmly on the counter to release large bubbles, and bake 23–27 minutes, rotating once, until a toothpick comes out with a few moist crumbs. Cool in pans 10 minutes, then invert onto racks, peel parchment, and cool completely. For tidier assembly, chill the layers 20 minutes to firm up.
• Start the buttercream: Place the chopped chocolate in a microwave-safe bowl and melt at 50% power in 20–30 second bursts, stirring between each, until smooth. Aim for 90–95°F (lukewarm), then set aside. In the bowl of a stand mixer, whisk the egg whites and sugar to combine. Set the bowl over a pot with 1–2 inches of barely simmering water and cook, whisking constantly, until the mixture reaches 160°F and no sugar grains remain, 6–8 minutes.
• Whip and finish the frosting: Move the bowl to the mixer fitted with the whisk and whip on medium-high until the meringue is glossy, voluminous, and the bowl is cool to the touch, 8–10 minutes. Switch to the paddle and add butter a tablespoon at a time on medium speed until thick and silky; it may briefly look curdled before coming together. Mix in the vanilla, then stream in the melted chocolate on low. Beat 2–3 minutes more until fluffy and smooth.
• Assemble: Level cakes if needed. Place the first layer on a board and add a very thin smear of buttercream to anchor the filling. Using a spoon or piping bag, build a 1/2-inch rim of buttercream around the edge. Spread half of the chilled strawberry filling inside the rim. Top with the second layer and repeat with the remaining filling. Set the final cake layer on top and chill the stacked cake for 15 minutes.
• Finish and decorate: Apply a thin crumb coat of buttercream and chill 10 minutes. Add a final coat, smoothing the sides and creating soft swoops on top. Garnish with fresh strawberries just before serving. For the cleanest slices, warm a knife under hot water, wipe dry, and cut, wiping between slices.