Preheat your oven to 204 °C (200 °C). Generously grease two 6-ounce ramekins with softened butter, ensuring complete coverage. Dust the inside of each ramekin with cocoa powder, tapping out any excess. Place the prepared ramekins on a baking sheet.

In a heatproof bowl set over a saucepan of simmering water (double boiler method), melt the chopped bittersweet chocolate and butter together. Stir occasionally until smooth and fully combined. Alternatively, microwave in 30-second intervals, stirring between each, until melted and smooth. Remove from heat and let cool slightly.

In a separate medium bowl, whisk together the eggs, granulated sugar, espresso powder (if using), vanilla extract, and salt until the mixture is pale and slightly frothy. This should take about 2-3 minutes with a hand whisk.

Gently fold the slightly cooled chocolate mixture into the egg mixture until just combined. Be careful not to overmix. Sift the flour over the batter and fold it in until no streaks of flour remain.

Evenly divide the batter between the two prepared ramekins. They should be about two-thirds full.

Bake for 12-14 minutes. The edges should be set and slightly puffed, while the center should still appear soft and jiggly. Do not overbake, as this will cook the center through.

Carefully remove the ramekins from the oven and let them rest for 2-3 minutes. This allows the edges to firm up slightly. To unmold, run a thin knife around the inside edge of each ramekin, then invert onto serving plates. If needed, give the ramekin a gentle tap to release the cake.

Serve immediately. Garnish with fresh raspberries and a light dusting of powdered sugar, if desired. A small dollop of whipped cream or a scoop of vanilla ice cream is also a delightful accompaniment.