Preheat the oven to 390 F. Line to baking sheets with parchment paper.

Melt the butter and 250g of the dark chocolate in a microwave or over a double-boiler. If using a microwave, heat at 30 second intervals, stir, then repeat until butter is melted and chocolate is not quite melted. Stir rapidly to finish melting the chocolate. Let cool slightly to room temperature.

Whisk together the eggs and brown sugar until the mixture lightens and increases substantially in volume (by about double), about 5 minutes. Fold the melted chocolate-butter mixture into the egg yolk-sugar liquid. Sprinkle with the cocoa powder and remaining 50 g of chopped dark chocolate, fold to combine.

Using two spoons or a piping bag, make 14 rounds of cookie dough (30-35 g each) and place on the lined baking sheets. Allow 2-inches between each, as cookies will spread significantly during baking.

Bake for 7-9 minutes, until cookies have formed a crust but remain quite soft. Do not overbake.

Remove from the oven, cool on the sheet 5-10 minutes, then transfer to a wire cooling rack to cool completely.

Place all the ingredients into the mixing bowl of a stand mixer and mix at very low speed. Alternately, whisk together by hand, being careful not to over-whisk. Filling should be smooth, creamy, and stiff enough to hold a shape.

Flip the cookies bottom-up, pair with a similar-sized cookie. Place a spoonful of filling in the center of half the cookies, then close the sandwich with the other half of the cookie pair. Press lightly to push the filling just to the edges.

Serve or keep in the refrigerator up to 3 days.