Place the butter and granulated sugar in the bowl of an electric mixer with the paddle attachment in place. Beat on medium-high speed until light and fluffy, then gradually add the egg and continue to beat until incorporated. Sift the flour, baking powder, cocoa powder, cinnamon and salt into a bowl, then add to the butter and sugar. Mix on low speed for about 15 seconds, then add the chocolate chips and banana. Beat until combined, then transfer to the fridge for 2 hours to firm up.
When firm, use your hands to form the dough into 1-inch/3-cm round balls, about 2/3 oz/20 g each; you might need to wash your hands once or twice when making them, if they get too sticky. Place the pecans in a medium bowl and drop the balls into the nuts as you form them, rolling them around so that they are completely coated and pressing the nuts in so that they stick.
Line a baking sheet with parchment paper, place the cookies onto the sheet—there is no need to space them apart—and transfer to the fridge for an hour.
When ready to bake, preheat the oven to 375°F/190°C. Line two baking sheets with parchment paper.
Place the confectioners’ sugar in a bowl and roll the cookies in the confectioners’ sugar, pressing it in as you go so that it sticks well. Place on the lined baking sheets, spaced 1 inch/2.5 cm apart, and flatten the cookies to 1/3 inch/1 cm thick.
Bake for 10 minutes. They will be soft to the touch when they come out of the oven, so allow them to cool on the baking sheet for 10 minutes before gently transferring to a wire rack. These can be served warm, when they will be a little gooey in the center, or set aside until completely cool.