Preheat the oven to 180°C for 15–20 minutes using top and bottom heat mode. For accuracy, use an oven thermometer.
Brown the butter over low heat until dark golden brown, with milk solids forming at the bottom of the pan. Set aside to cool.
Combine the flour, cocoa powder, baking soda, and baking powder. Sift together and set aside.
Crumple a sheet of parchment paper and use it to line the baking tin.
In a mixing bowl, mash the banana. Add the Greek yoghurt, egg, cinnamon, vanilla extract or essence, and browned butter. Mix until well combined.
Fold in the flour mixture until just combined, i.e. no dry flour or lumps remaining. Do not overmix.
Fold in the diced apples until evenly distributed.
Bake at 180°C for approximately 45 minutes, or until a skewer inserted into the centre comes out clean.
Cool, slice, and serve.