Preheat your oven to 150°C (300°F) and line a loaf pan with parchment paper.
Melt the dark chocolate using a double boiler or in the microwave in short bursts until smooth. Set aside to cool slightly; it should be warm, not hot.
In a mixing bowl, whisk the melted chocolate and vegetable oil together until smooth. Add the milk and mix well. Sift in the cake flour and cocoa powder, then stir until just combined.
Add the egg yolks and whisk until the batter is smooth, glossy, and free of lumps. Set aside.
In a clean, grease-free bowl, combine the egg whites and lemon juice. Beat until foamy.
Add the sugar and whip until stiff peaks form.
Add about one-third of the meringue to the chocolate batter and whisk until fully incorporated. This helps lighten the batter and makes folding easier.
Pour the chocolate batter into the remaining meringue. Using a rubber spatula, gently fold until no white streaks remain. Be careful not to overmix, as this can deflate the batter.
Pour the batter into the prepared loaf pan. Gently tap the pan on the counter a few times to release any large air bubbles.
Bake for 40 minutes, or until the top looks set.
Remove the cake from the oven and quickly arrange the chocolate squares on top. Return it to the oven and bake for another 10 minutes, or until a toothpick inserted into the cake comes out clean.
Let the cake cool in the pan on a wire rack for about 10 minutes.
Carefully lift the cake out using the parchment paper, peel away the paper, and allow it to cool completely before slicing and serving.
