Preheat your oven to 325°F (160°C).

Spray a 9-inch nonstick springform pan with cooking spray. Line the bottom with a circle of parchment paper and spray the top of the paper as well.

In a double boiler or heatproof bowl set over (not in) a pot of boiling water, melt the chopped chocolate and cubed butter. Stir gently to avoid burning, then set aside to cool for a few minutes.

In the bowl of a stand mixer, use the whisk attachment to beat the granulated sugar, brown sugar, and eggs together on medium-high speed until fluffy (about 8 to 10 minutes).

With the mixer on low, gradually add the cocoa powder, vanilla extract, and kosher salt. Slowly drizzle in the cooled chocolate mixture, mixing until just combined.

Pour the batter into the prepared pan and smooth the top. Gently tap the pan on the counter a couple of times to release any trapped air bubbles.

Bake for approximately 50 minutes or until the top is just set. The center should be a bit wobbly but not liquid.

Allow the cake to cool completely before chilling in the refrigerator for at least 2 hours or ideally overnight.

When ready to serve, carefully remove the cake from the pan and place it on a serving platter. Sprinkle the top with sifted powdered sugar for a beautiful finish.