Preheat oven to 350°. Spray a 10" springform pan with baking spray, then seal the base of the pan in foil to prevent water from leaking in.
Melt chocolate and butter together in a double boiler or in the microwave. If using the microwave, cook on high in 30 second increments until it is completely smooth. Let cool for 5 minutes.
Whisk sugar and eggs together in a large bowl until fully combined. Slowly pour chocolate over mixture stirring quickly until fully combined.
Pour the mixture into the springform pan, cover tightly with foil and place into a baking pan. Fill the baking pan with very hot water until it is about 1" high. Bake for 75 minutes.
Remove from the water bath and allow it to cool completely before serving.
Garnish with raspberries and powdered sugar or whipped cream, as desired.