Preheat the oven to 180 °C (356 °F) and place a rack in the center. Lightly grease madeleine molds or muffin cups with a little of the room-temperature butter; if you like, dust the greased molds lightly with a pinch of the flour.

If your chocolate is not already melted, melt the 100 g chocolate gently (microwave in short bursts or over a double boiler) and let it cool until warm but not hot (too-hot chocolate can cook the eggs).

In a large bowl, beat the 100 g room-temperature butter and 100 g sugar together on medium speed until pale and creamy, about 2–3 minutes.

Add the 3 eggs one at a time, beating after each addition until fully incorporated. Add the 1 tbsp vanilla extract and mix briefly to combine.

With the mixer on low, pour in the warm melted chocolate and mix until evenly combined.

In a separate bowl, whisk together the 150 g flour, 10 g baking powder, and the pinch of salt.

Fold the dry ingredients into the wet mixture gently with a spatula until just combined and the batter is smooth and slightly airy. Do not overmix.

Spoon the batter into the prepared madeleine molds or muffin cups, filling each about two-thirds full to leave room for rising.

Bake for 10–15 minutes (madeleines) or longer if using muffin cups, until the tops are golden and a toothpick inserted into the center comes out with a few moist crumbs.

Remove from the oven, let the cakes rest in the pan for 1–2 minutes, then transfer them to a wire rack to cool. Cool completely before serving.