Preheat the oven to 350°F. Butter a 9-inch springform pan and line the bottom with parchment paper. Wrap 3 layers of heavy-duty foil around the outside of the pan, ensuring it reaches the top of the rim.

In a small saucepan, combine water and sugar. Bring to a boil over medium heat, stirring until the sugar dissolves. Simmer for 5 minutes, then remove from heat.

In a large saucepan, melt butter over low heat. Add the chopped chocolate and whisk until smooth. Whisk the sugar syrup into the chocolate mixture, then let it cool slightly.

Add the eggs to the cooled chocolate mixture, whisking until well blended.

Pour the batter into the prepared pan. Place the pan in a large roasting pan and add enough hot water to the roasting pan to come halfway up the sides of the cake pan.

Bake for about 50 minutes, until the center no longer jiggles when gently shaken. Remove from the water bath and cool completely in the pan.

For the ganache: Bring the heavy cream to a simmer in a small saucepan. Remove from heat, then add the chopped chocolate and whisk until smooth.

Pour the ganache over the cooled cake while still in the pan. Gently shake the pan to evenly spread the ganache. Refrigerate the cake for about 2 hours until the ganache is set.

Dust the top with cocoa powder just before serving.