Start by scooping pistachio paste into silicone molds or an ice cube tray. Level it off and place it in the freezer for at least one hour.

While the pistachio filling firms up, set up a double boiler. In a heatproof bowl, combine dark chocolate and unsalted butter, stirring until melted and smooth.

In a large mixing bowl, whisk together eggs and granulated sugar until thick and pale, approximately 3-5 minutes. Gently fold the melted chocolate mixture into the eggs.

Preheat your oven to 425°F (220°C) and grease four ramekins with butter. Fill each ramekin one-third full with batter, place the frozen pistachio filling in the center, then cover with more batter.

Carefully place the filled ramekins on a baking tray and bake for 12-13 minutes. The tops should be set and slightly cracked but still soft in the middle.

Let the cakes cool for a minute, then gently invert each ramekin onto a plate and serve immediately.