Prepare the grill for indirect cooking over medium heat (about 350°).
In a medium saucepan over low heat, melt the chocolate chips and butter, stirring often (be careful not to burn the chocolate). Remove the pan from the heat.
In a large bowl using an electric mixer, beat the sugar and eggs on medium-high speed until pale yellow and thick, 4 to 5 minutes.
In a large bowl sift the flour, cocoa powder, cinnamon, baking soda, and salt.
Grease the inside of a 9-inch round cake pan with butter.
With the electric mixer on low, add the flour mixture to the sugar-egg mixture. Then add the chocolate mixture. Mix until just smooth. With a rubber spatula transfer the batter to the cake pan, spreading it evenly.
Brush the cooking grates clean. Cook the cake over indirect medium heat, with the lid closed as much as possible, until a skewer inserted in the center comes out with moist crumbs attached, 35 to 45 minutes, rotating the pan 90 degrees after about 20 minutes. Remove from the grill and let cool in the pan completely, about 1 hour. Invert the cake onto a serving platter. Carefully turn it over. Sift the confectioners’ sugar through a sieve on top of the cake. Cut into wedges. Serve at room temperature.