Preheat your oven to 180°C (350°F). Allow the oven to fully preheat for at least 10 minutes to ensure an even bake. Prepare a loaf pan by lining it with parchment paper, allowing some overhang on the sides for easy removal later.

Ensure you have 150g of steamed Kabocha squash. If you are starting from fresh, wash the squash, cut it into 1-2cm cubes, and steam for 15-20 minutes until very tender. Let it cool, then mash it until smooth.

In a large mixing bowl, combine the 150g of Kabocha squash and the 10g of sweetener (if using). Mash until smooth.

Add the 2 room-temperature eggs to the bowl. Pour in the 70g of whole milk and whisk until the liquid ingredients are well combined and smooth.

Sift the 80g of cake flour and the 3g of baking powder directly into the wet ingredients.

Whisk until no dry flour remains. Be careful not to overmix.

Add the 30g of instant oats and most of the chopped, toasted nuts and chestnuts to the batter (reserve some for the top). Gently fold them in until they are evenly distributed.

Pour the finished batter into your prepared loaf pan. Use a spatula to smooth the top into an even layer.

Artfully arrange the reserved 4 thin Kabocha slices, 3-4 whole chestnuts, and reserved chopped nuts on top of the batter. Gently press them down slightly so they adhere but are still visible.

Place the pan in the preheated oven on the middle rack. Bake for 30-35 minutes, or until a toothpick inserted into the center of the cake comes out with a few moist crumbs attached. If the toothpick comes out with wet batter, bake for an additional 2-3 minutes and test again.

Once baked, remove the pan from the oven and place it on a wire cooling rack. Let the cake cool in the pan for at least 20 minutes. Then, use the parchment paper overhang to carefully lift the cake out of the pan and onto the rack to cool completely. This cooling period is important for the texture to set properly. Slice and serve.