Prepare the grill for indirect cooking over medium heat (about 180ºC).
In a medium saucepan over very low heat on the stove, melt the chocolate chips and the butter pieces, stirring often (be careful not to burn the chocolate). Remove the pan from the heat.
In a large bowl using an electric mixer on medium-high speed, beat the sugar and eggs until pale yellow and fluffy, 4 to 5 minutes.
In another large bowl sift the flour, cocoa powder, cinnamon, baking soda, and salt.
Grease the inside of a 23-centimeters round cake pan with softened butter.
With the electric mixer on low speed, add the flour mixture to the sugar-egg mixture. Add the chocolate mixture and mix until just smooth. With a rubber spatula transfer the batter to the cake pan, spreading it evenly.
Cook the cake over indirect medium heat, with the lid closed, until a wooden toothpick 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 completely in the pan on a wire rack, about 1 hour. Invert and unmold the cake onto a serving platter. Carefully turn the cake over. Sift the confectioners' sugar through a fine-mesh sieve over the top of the cake. Cut into wedges. Serve at room temperature.
Cook the cake over medium heat, with the lid closed, until a wooden toothpick 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 completely in the pan on a wire rack, about 1 hour. Invert and unmold the cake onto a serving platter. Carefully turn the cake over. Sift the confectioners' sugar through a fine-mesh sieve over the top of the cake. Cut into wedges. Serve at room temperature.
