Preparation: Line the base and sides of a 7.5 or 8-inch springform pan with parchment paper. I use an 8-inch pan in the images. Preheat the oven to 160°C (320°F) fan-forced, or 180°C (356°F) conventional.

Crumb topping and filling: Add all the crumble ingredients to a mixing bowl. Use your fingertips to rub the mixture together until it forms a sandy, crumb-like texture. Set aside.

Mix dry ingredients: Sift the flour, baking powder, baking soda, sea salt, and cinnamon into a bowl and stir until combined. Set aside.

Puree sweet potato: Once cooled, peel the sweet potato and weigh out the flesh - you need 350g. Add it to a food processor with the vegan yogurt, vanilla, and zest and blitz until smooth.

Mix wet ingredients: In a large bowl, whisk together the sweet potato mixture, brown sugar, olive oil, and vegan buttermilk until smooth and well combined.

Make the batter: Add the dry ingredients to the wet in two additions, gently folding after each one until just combined. Be careful not to overmix.

Assemble: Spread half of the batter evenly into the prepared pan, smoothing it out with a spatula. Sprinkle half of the crumble mixture over the batter in an even layer. Gently dollop the remaining batter on top, then carefully spread it out to cover the crumble. Finish by scattering the remaining crumble over the surface.

Bake: Bake for 65 to 75 minutes, or until a skewer inserted into the center comes out mostly clean with just a few moist crumbs. If there’s wet batter on the skewer, return the pan to the oven and bake in 5-minute intervals, checking again as needed.Once baked, allow the cake to cool in the tin for 15 minutes. Then carefully lift it out and transfer to a wire rack to cool completely. Just before serving, dust with powdered sugar for a beautiful finish.