Preheat your oven to 425°F (220°C).

Generously butter four ramekins and dust them with flour to prevent sticking. Tap out any excess flour.

In a microwave-safe bowl, combine the unsalted butter and chopped dark chocolate. Microwave in short bursts, stirring often, until melted and smooth.

Add the powdered sugar to the melted chocolate mixture, stirring until well combined. Incorporate the eggs and yolks one at a time, mixing after each addition. Add the vanilla extract, then gently fold in the flour and the chopped pistachios until just combined.

Divide the batter evenly among the prepared ramekins, filling each about 3/4 full.

Place the ramekins on a baking sheet and bake for 12-14 minutes, or until the edges are firm but the center remains soft.

Let them cool for a minute before carefully inverting each ramekin onto a plate. Dust with powdered sugar if desired, and serve warm.