Preheat oven to 350°F
Prepare an 8-inch round spring form or cake pan. If using a spring form, butter or spray the pan with baking release spray. If using a cake pan, line the pan with a piece of aluminum foil large enough to hang the edges over the pan. Spray the aluminum foil with baking release spray or butter it with your hands.
In a large bowl of an electric mixer, beat the butter with the sugar for several minutes until blended and creamy.
Add the eggs and vanilla and beat until well blended.
Combine flour, salt and baking powder.
Add the dry ingredients and beat on medium speed until just blended.
Break the chocolate into equal size pieces and place in a microwave-safe bowl.
Microwave at high for 20 seconds. Remove the microwave and stir. Repeat once more. After the second microwaving, the chocolate should mostly be melted with a few solid pieces. If you can stir the chocolate to melt the remaining chocolate, do that. If the remaining chocolate is not melting, zap it again for 10 seconds. You want to microwave it as little as possible to get it melted.
Let the chocolate cool for 2-3 minutes until it is just warm to the touch.
Add the melted chocolate to the cake batter and beat on medium speed until just incorporated.
Pour the batter into the prepared pan and smooth the surface with a spatula.
Bake for 35 - 40 minutes until a tester comes out without batter but with a small crumb. Don't overbake!
Remove and allow to cool for 20-30 minutes on a wire cooling rack.
If using the aluminum foil in a cake pan, lift the cooled cake out of the pan by the edges of the foil. Turn the cake, face side down, and remove the foil. Then turn the cake right side up to serve.
Serve as is, dusted with powdered sugar or topped with fresh berries.