adapted slightly from kingarthurflour.com
8 oz. (1 cup) semi sweet chocolate chips
1/2 cup butter
3/4 cup white sugar
1/4 teaspoon salt
1 teaspoon vanilla
3 eggs, beaten
1/2 cup unsweetened cocoa powder
1) Preheat the oven to 350°F. Lightly grease an 8" round cake pan or springform pan; cut a piece of parchment or waxed paper to fit, grease it, and lay it in the bottom of the pan.
2) Put the chocolate and butter in a microwave-safe bowl, and heat until the butter is melted and the chips are soft. Stir until the chips melt, reheating briefly if necessary. Transfer the melted chocolate/butter to a mixing bowl.
3) Stir in the sugar, salt, and vanilla.
4) Add the eggs, beating briefly until smooth. Add the cocoa powder, and mix just to combine.
5) Pour the batter into the prepared pan.
6) Bake the cake for 25 minutes; the top will have formed a thin crust.
7) Remove it from the oven, and cool. (Its easiest to cut after being refrigerated.)