Sift cake flour and baking powder together; set aside. Cream unsalted butter until smooth. Add granulated sugar in 2 parts. Mix well each time.

Add salt, and mix well. After adding the salt, mix well with a spatula for 3-4 mins. Please mix while rubbing and stretching.

Add egg yolk, and mix well until combined.

Sift almond flour over the batter, and mix well until smooth.

Add rum and vanilla oil, and mix well until smooth. Please mix gently at the beginning so that the rum doesn't splash.

Add dry ingredients. Mix it while cutting it, scooping sometimes. Mix it until the large lumps form like this (3rd Photo).

Place the dough on plastic wrap and press lightly to flatten while gather the lumps.

Fold in half and press it. Repeat folding and pressing 5-6 times until smooth.

Shape it square and wrap it with plastic wrap. Let it sit in a fridge for 3 hours (If possible, overnight).

Take the dough out of the fridge, roll it out into 1.2cm (0.47'') thick while coating it lightly with bread flour. If your dough is hard, please leave it for about 15 mins at room temperature to soften a little. Cut it with a 6 cm (2.4'') mousse ring. Re-roll the dough scraps and cut again. (If it gets too soft, please cool it once.)

【For Brushing】 Mix egg yolk and 1/2 tsp water, and strain to smooth.

Brush the top with the egg wash, with a little space on the edge. Let it sit in a fridge for 30 mins to dry the egg wash. Preheat an oven to 190℃ / 374°F. Grease inside of mousse rings with butter.

Using a fork, create a crosshatch pattern over the dough circles. Transfer the circles onto a parchment paper-lined baking tray. Fit mousse rings into circles. Remove the outside extra dough. (I have only 5 rings, so I baked them in 2 parts.)

Bake it at 170℃ / 338 F for25-30 mins until golden brown. Remove the rings gently while cookies hot. Let it cool thoroughly on a rack. DONE!!