03For the buttercream: Place the cream and chocolate chips in a medium bowl over a pan of barely simmering water. Stir until the chocolate has melted and the mixture is smooth. Remove the bowl from the pan and refrigerate until thick, 20 minutes. Add the butter and cocoa powder and beat the mixture on high speed until light and fluffy, about 1 minute. Spread the buttercream over the cooled brownie layer. Dust with the cinnamon and refrigerate until firm, about 30 minutes.04Lift the brownie layer from the pan, using the excess paper for handles. Run a thin spatula underneath the brownie layer to loosen it from the parchment paper. Cut into 24 brownies and serve.05Cook's Note: The buttercream can be made ahead of time. Cover and refrigerate until ready to serve. Allow the buttercream to come to room temperature for 30 minutes then beat until smooth enough to spread.