Lightly grease a 9-inch round cake pan with butter.
In a medium bowl, whisk together 1/2 cup sugar, cocoa powder, 4 tablespoons butter, flour, baking powder, and salt. Stir in the milk, egg yolks, and vanilla extract until combined.
In a separate bowl, beat the egg whites with an electric mixer until stiff peaks form. Slowly add the remaining 1/4 cup of sugar and continue beating until glossy.
Gently fold the egg whites into the chocolate mixture until just combined. Pour the batter into the prepared pan.
Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
Meanwhile, make the mocha sauce by combining the dark chocolate chips, remaining 2 tablespoons butter, hot coffee, and cinnamon in a medium saucepan. Heat over low heat, stirring occasionally, until the chocolate chips are melted and the sauce is smooth.
Remove the souffle from the oven and let cool for 10 minutes before serving. Serve warm with the mocha sauce.
Interesting Facts
Souffles were invented in 17th century France.
The name "souffle" comes from the French word for "puff".
This dessert is perfect for a romantic Valentine's Day celebration.