Cut the butter into pieces then place into a saucepan & set over a medium heat. Cook, stirring regularly until brown then take off the heat & stir in the toasted milk powder (if using). Pour the butter into a bowl & set aside to cool slightly.
Preheat an oven to 175°c/155°c fan (350°f/310°f).
Line a 9" x 13" baking tin with baking parchment then set aside.
Place the banana into a bowl, mash with a whisk then add in the brown butter, brown sugar, black treacle & vanilla. Whisk well, to combine.
Add the eggs & sour cream into the banana mix then whisk again, to incorporate.
Next, sift the flour, baking powder, bicarbonate of soda, cinnamon & ginger into the batter, add in the salt then whisk gently, until the flour is just incorporated.Give the sides of the bowl a quick scrape down with a spatula, to make sure all the flour has been incorporated! Once mixed, immediately pour the batter into the lined tin then bake in the preheated oven for 40-45 minutes, until a skewer inserted into the centre comes out clean.
Let the sponge cool in the tin before cutting into portions or store in the fridge for up to 3 days.