Scoop 1 teaspoon of chocolate spread onto a small piece of parchment paper. Form it into a small ball or mound and place it in the freezer for at least 15 minutes.
Preheat the oven to 200°C (392°F). Generously butter and dust a small ramekin or mold with flour or cocoa powder, tapping out the excess.
Place the butter and bittersweet chocolate in a heatproof bowl. Set the bowl over a saucepan of gently simmering water (double boiler). Stir until smooth, then remove from heat and let cool slightly.
In a separate bowl, whisk the egg, egg yolk, and granulated sugar until pale and slightly thickened (2–3 minutes).
Gradually pour the melted chocolate into the egg mixture, whisking constantly to prevent the eggs from scrambling. Mix until smooth and fully incorporated.
Sift in the flour and gently fold it into the batter until no lumps remain. Avoid overmixing to keep the batter light.
Fill the prepared ramekin about ¾ full with batter. Gently press the frozen chocolate spread ball into the center of the batter, ensuring it is submerged but not touching the bottom.
Place the ramekin on a baking sheet and bake for 8–10 minutes, or until the edges are set and the center is slightly jiggly.
Remove from the oven and let the cake rest for 1–2 minutes. Carefully run a knife around the edges and invert the cake onto a serving plate. Serve immediately with a scoop of vanilla ice cream or fresh berries.
