Preheat the oven to 180°C (350°F). Line a 20 cm (8 inch) round cake tray with baking paper.
To make the cake: Add all the dry ingredients to a large mixing bowl. Mix until there are no lumps. Add all the wet ingredients to a blender and blend until mostly smooth. Pour the mixture into the bowl and mix until combined. Pour the cake batter into the cake tin. Flatten the top with a spatula.
To prepare the crumble topping: In a medium bowl, combine all the ingredients (except the papaya) and mix until combined and crumbly.
Scatter the papaya on top of the cake and gently press them into the batter. Scatter the crumble on the cake.
Bake the cake in the oven for around 50 minutes or until a skewer can be inserted into the middle and there is no wet batter on it. The cake is quite moist and dense so there might be some crumbs on the skewer which is fine. If the cake is browning too quickly at any point, cover it with an oven-safe plate or aluminium foil.
Allow the cake to rest in the cake tin for at least 5 minutes. Remove the cake from the tin and allow to cool. Serve and enjoy!
The cake can be kept at room temperature for 1 day or in an airtight container in the fridge for 3 days.
*For a gluten free papaya cake, substitute the plain flour for 1 ½ cups (150g) almond meal and ¾ cup (120g) fine white rice flour.