Thoroughly peel the sweet potato, cut into 2 cm cubes, and soak in water. Add to a heat-resistant container, wrap, and microwave to soften. While they're hot send through a strainer, cool, and set aside. Leave out the butter at room temperature.
Add the butter to a bowl and use a whisk to whip into a cream. Add the sugar in 3 parts and mix until it becomes white-ish. Add the beaten egg a little at a time and mix. Add the sweet potatoes and mix well. Set aside 50 g of the dough in a different bowl (for making the cocoa dough).
Combine and sift the cake flour, baking powder, and cinnamon powder and add to the bowl in step 2. Use a rubber spatula to mix well. Once it's become a unified mass wrap it up and refrigerate for 30 minutes.
Now make the cocoa dough. Combine and sift the cake flour and cocoa and add to the bowl you set aside in step 2. Use a rubber spatula to mix well. Once it's become a unified mass wrap it up and refrigerate for 30 minutes.
Take the dough from step 3 and make into a 15 cm round stick. Place the cocoa dough on top of wrap, then another layer of wrap on top of the cocoa. Use a rolling pin to thin it out. Encase the sweet potato dough with the cocoa dough. Wrap it up and cool in the freezer.
Remove from the freezer before it completely freezes (hard enough to be cut with a knife). Cut diagonally into 5 mm slices. Place on a baking pan, coat with water-dissolved egg yolk, and sprinkle with sesame. Bake in your preheated oven at 170℃ for 20 to 25 minutes.
Story Behind this Recipe
I wanted to eat a sweet potato cookie.
When covering the sweet potato dough with the cocoa dough, to ensure it doesn't come off when baking be sure to stick it on tightly by rolling it around when wrapped up. If you're worried about it then use some egg whites as a sort of paste, which keeps it from falling apart. These can be stored in the freezer from step 5.