Preheat the oven to 200C/180C Fan/Gas 6. Line an 8 inch square tin. Melt the dark chocolate in the microwave in 20 second intervals. Set aside.
In a large bowl/the bowl of a stand mixer, combine the melted butter and both sugars. Add the eggs and mix again. Slowly incorporate the melted chocolate until everything is fully combined.
Fold in the flour, cocoa powder and chopped Lindt chocolate until just combined.
Pour the batter into the prepared tin. Bake for 30-35 minutes, then let cool on a wire rack.
Optional: Drizzle over some melted white chocolate and top with extra chocolate eggs if desired!