Preheat oven to 350°F. Butter 10-inch-diameter springform pan. Line bottom with waxed paper. Stir butter and chocolate in heavy large saucepan over low heat until melted. Add sugar and cocoa and whisk to combine. Add eggs; whisking until well blended. Pour batter into prepared pan. Bake until tester inserted into center comes out clean, about 45 minutes. Cool cake completely in pan on rack. Run knife around pan sides to loosen cake. Release pan sides.