1. Preheat the oven to 180 degrees Celsius.

2. Grease a cake pan with butter and dust it with flour.

3. In a saucepan over medium heat, melt the milk and chopped bittersweet chocolate. Stir occasionally until the chocolate is completely melted.

4. Remove the saucepan from the heat and stir in vanilla extract and sugar until the sugar is dissolved.

5. In a separate bowl, mix the flour and cinnamon together.

6. Slowly pour the melted chocolate mixture into the flour mixture and stir well until smooth batter forms.

7. Pour the batter into the prepared cake pan and smooth the top.

8. Bake the cake in the preheated oven for 30-35 minutes or until a toothpick inserted into the center of the cake comes out clean.

9. Remove the cake from the oven and let it cool. Dust with powdered sugar or decorate with chocolate frosting, if desired.