Have the butter cut into 1 cm cubes and frozen. Put the butter and the sifted dry ingredients into a food processor and process until the whole thing becomes grainy.
Add the ingredients marked with ☆ to the food processor and blend.
Turn out the dough onto a work bench and knead it a bit. Then shape it into a round disk and cover with plastic wrap and chill in the fridge for at least 1 hour (prepare it on the day before if you can).
Cut the sweet potato into bite-sized chunks and soak in water. Then drain it, add the milk, cover it with plastic wrap, and microwave it for 8-10 minutes. Mash the potato afterward.
Add the margarine, the sugar, and the egg yolk in that order to Step 4 and mix thoroughly after each addition. Finally, add the cream and mix well.
Turn out the chilled dough onto a work surface dusted with flour and roll it out to a thickness of 4-5 mm. Line the tart pan with the dough, and cut off the excess dough with a knife (you can freeze the leftovers).
Dock the dough, cover with aluminum foil, put a pie weight on it, and bake in an oven preheated to 180℃ for 10-15 minutes. Let the crust cool after it has been baked.
Fill the tart crust (Step 7) with the filling (Step 5). Brush on some egg wash (egg yolk mixed with water), and sprinkle on some black sesame seeds if you desire.
Bake for 20 minutes at 190℃, then raise the temperature to 220℃ and bake for about 10 more minutes (adjust baking time as to not burn it).
When it's done baking, place on a wire rack and cool. Take it out of the tart pan once it has cooled.
Cut up into your desired size.
Depending on the oven you use, baking time and temperatures may vary. Please pay attention and adjust accordingly.
If you don't have heavy cream, you can substitute it with milk, but it'll turn out more moist and tasty with the cream.
Adjust the amount of sugar depending on how sweet the potato is.
You can make this with an toaster oven as well: https://cookpad.com/en/recipes/143577-easy-delicious-sweet-potato-cakes