Chop chocolate into small pieces

Stir chocolate and butter in a heavy large saucepan over low heat until melted and smooth

Cool 5 minutes

Preheat oven to 325 degrees

Butter an 8-inch round cake pan and line with parchment paper

Whisk sugar, cinnamon and salt into cooled chocolate mixture

Whisk in eggs, one at a time, then vanilla

Continue to whisk until batter is smooth

Add flour and stir just until blended

Stir in chocolate chips

Pour batter into prepared pan, smoothing surface

Bake until tester inserted into center comes out with a few moist crumbs attached, 25 to 30 minutes

Cool completely in pan on rack

Cut into wedges to serve with ice cream.