Preheat oven to 180 degrees Celsius (300 degrees Fahrenheit), and line a square baking tin with baking paper.

In a medium sized bowl, mix together the butter, banana and cinnamon with a wooden spoon. Add the Greek yogurt and stir until no lumps remain, and then add the milk and sugar, stirring to combine.

Incorporate into this the flour, baking powder and salt, and stir to completely combine, using a spatula to get any flour from the sides of the bowl. Fold in the chocolate chips until just incorporated.

Spread the batter into the baking tin and bake for 20 minutes, or until the top is golden. Cool to room temperature before cutting into even pieces. For a fudgy texture, allow cooling for 3 hours before serving.