MAKE THE PISTACHIO FILLING: In a heatproof bowl, melt the white chocolate with heavy cream using a double boiler or microwave in 20-second intervals, stirring until smooth. Stir in the pistachio paste and a pinch of salt until fully combined and smooth. Spoon the mixture into a silicone ice cube tray or small mold to create 6 equal discs. Freeze for at least 1 hour or until solid. These can be made a day ahead and stored in an airtight container in the freezer.

PREPARE THE RAMEKINS: Preheat the oven to 425°F (220°C). Grease six 6-ounce ramekins with butter and dust with cocoa powder. Place them on a baking tray for easy handling.

MAKE THE CHOCOLATE BATTER: In a small saucepan or microwave-safe bowl, melt the butter and dark chocolate together until smooth, stirring frequently. In a medium bowl, whisk the eggs, egg yolks, sugar, and salt until thick and pale, about 2 to 3 minutes. Slowly whisk in the melted chocolate mixture until fully incorporated. Gently fold in the flour until just combined, being careful not to overmix.

ASSEMBLE THE LAVA CAKES: Spoon about 2 tablespoons of chocolate batter into the bottom of each prepared ramekin. Place one frozen pistachio disc in the center of each. Cover with additional batter until each ramekin is approximately three-quarters full.

BAKE AND SERVE: Bake for 11 to 13 minutes, or until the edges are firm and the centers still jiggle slightly. Remove from the oven and let rest for 1 minute. Run a knife around the edges to loosen if needed, then carefully invert onto serving plates. Garnish with chopped pistachios if desired and serve immediately.