2sticks16 tablespoons unsalted butter, cut into cubes (plus more for greasing the pan)
Powdered sugarfor garnish
Preheat the oven to 325ºF. Line the bottom of a 9-inch springform pan with a circle of parchment paper. Grease the parchment paper and the sides of the pan with butter. Set the pan on top of a sheet of heavy-duty aluminum foil and wrap the foil up around the sides. Do this several times to ensure the pan is sealed around the bottom. Place the pan in another oven-safe pan large enough to hold the pan and some water, such as a sauté pan or deep skillet.
Place the eggs in a large bowl. Beat on high with an electric mixer until doubled in volume and frothy, about 4-5 minutes.
In a double boiler, melt together the chocolate chips and butter. Stir often, until melted and smooth. Remove from heat, set on a towel on the counter, and let cool for 2 minutes. Fold in ⅓ of the beaten eggs until just a few streaks remain. Add half of the remaining eggs and fold in again. Add the remaining eggs and fold in until thoroughly combined and no streaks remain. Work slowly as you fold to avoid deflating the eggs. Pour the batter into the prepared pan and smooth the top.
Place the pan/skillet holding the springform pan onto the middle oven rack. Very carefully pour boiling water into the pan/skillet that is holding the springform pan until the water comes halfway up the pan. Bake for 23-25 minutes, until shiny on top (like brownies) and an instant-read thermometer registers 140ºF when inserted into the center.
Remove the pan from the water and place on a wire rack to cool completely. Remove the foil. Cover the top with plastic wrap or foil and refrigerate for at least 4 hours or up to 3 days.
About 20 minutes before serving, carefully release the cake from the pan. Cut the cake into 10 slices, running the knife through hot water and wiping off between each cut. Garnish each slice with sifted powdered sugar, raspberries, and a few pistachios.