For the Streusel topping
Preheat the oven 160C/140C Fan/Gas 3. Grease and line a 900g/2lb loaf tin (20x12.5cm/8x5in).
Brown the butter in a pan by melting, then letting it foam and cook for 3-5 minutes, stirring occasionally until flecks of brown form at the bottom of the pan. Transfer it to a mixing bowl and whisk to let cool slightly. Note: this cannot be done with baking spread - just melt in the microwave.
Add the sugar to the butter and vigorously whisk together for a minute or so.
Add the bananas directly into the bowl and use a fork to mash.
Crack in the eggs and add the milk and vanilla. Whisk into a smooth mixture.
Sift in the flour, baking powder, bicarbonate of soda, cinnamon, cardamom and salt and stir to combine into a nice, thick batter.
Stir in the chocolate chips (or nuts) and spoon the mixture into the prepared loaf tin. Level the top.
To make the streudel topping add the plain flour, brown sugar, cinnamon and butter to a small bowl and rub in until the mixture looks like breadcrumbs. Mix in the walnut pieces.
Scatter the mixture evenly on top of the uncooked loaf and press on to the surface.
Bake for 52 minutes (1 hour if using the streusel topping) or until well risen, shrinking away from the sides of the tin and golden-brown.
Set aside to cool for 10 minutes, then remove from the tin and leave to cool on a wire rack.
