[Preparation]
Grease a cake pan with butter and line it with parchment paper.

Heat granulated sugar and unsalted butter to melt in a frying pan until golden brown.
Note: When you want to mix, please move the pan slowly without a spatula to prevent the sugar from turning into crystals.

Add the cut squashes and heat for 3-4 minutes while mixing with a spatula. Turn off the heat, add rum, and sift cinnamon over it. Stir gently.
Transfer it to a tray, spread it, and let it cool for 2-3 minutes.
Arrange squashes on the bottom of the cake pan, and pour the remaining sauce over. Set aside.

[Cake Batter]
・Preheat an oven to 200℃ / 392 F.
・Prepare boiling water to warm the egg mixture.
・Melt the butter in the microwave.

Break the eggs into a large bowl. Use a whisk to beat it lightly until smooth. Add granulated sugar and stir.
Pour hot water into a deep-sided container, and place the bowl in it. Warm it while stirring until 35°C / 95F, about body temperature.
Take the egg mixture out from the hot water, and put the container with butter and milk into the hot water. Set aside.

Use an electric mixer to beat the egg on high speed until whitish and heavy. It should fall like a ribbon and hold its shape for a moment.
Then beat it slowly on low speed for 1.5 mins to smooth it. It will become glossy.

Put the dry ingredients (flour, ground almonds, squash powder, baking powder, cinnamon powder) in a small bowl and mix until combined.
Sift the dry ingredients over the egg mixture in 2 batches, and fold it 20-30 times until you don’t see any flour left each time.

Remove the butter and milk mixture from the hot water.
Add a small amount of the cake batter to the milk mixture, and stir well until smooth. Put the mixture back into the cake batter, like drawing circles. Fold it 30 times to mix well.

Pour it over the caramel squashes. Lower the oven to 170 ℃ / 338 F and bake for 50 mins. Immediately drop the pan lightly onto your work surface to prevent the cake from shrinking. Let it cool as it is for 10-15 mins.

Cover it with a plate on the top, and turn it upside down to remove the cake.