Preheat the oven to 180C (350F).

Grease a 20.5cm round cake tin with oil and line it with greaseproof paper.

Sift the flour and baking powder into a large bowl.

Mix in the brown sugar.

In a separate bowl, stir together the mashed bananas, grated carrots, walnuts, beaten eggs, and sunflower oil.

Beat until the mixture is runny.

Pour the mixture into the prepared tin.

Bake in the preheated oven for 45-60 minutes, or until a skewer inserted into the center comes out clean and the cake shrinks slightly from the edges and is golden brown.

Let the cake cool in the tin for 10 minutes.

Turn it out onto a wire rack to cool completely.

Remove the lining paper.

Serve the cake cut into slices.