Add 3 tablespoons of water and ½ cup of packed brown sugar to a small saucepan. Whisk over medium-low heat until the sugar dissolves completely.

Transfer to a large bowl and set aside to cool to lukewarm.

Wet your hands slightly and divide the 8.5 oz of sweet red bean paste into 12 equal balls, about 20g (1 scant tablespoon) each. Set aside on a plate.

In a small dish, combine ½ teaspoon baking powder with ½ teaspoon water and stir. Add to the cooled brown sugar syrup and mix.

Sift 1 cup of cake flour into the bowl. Combine with a silicone spatula, chopsticks, or your fingers until a soft dough forms. Do not knead at this stage.

Cover with plastic wrap and rest for 15 minutes.

Transfer the dough to a floured surface and knead gently for 1 to 2 minutes, dusting with flour as needed. The dough should feel pliable and smooth. Do not over-knead.

Roll the dough into a log and cut into 12 equal pieces. Cover with a damp towel to prevent drying.

Take one piece of dough, form it into a ball, and flatten into a disc about 2.5 to 3 inches (6 to 7 cm) in diameter. Keep the center slightly thicker and the edges thinner.

Place one anko ball in the center. Pull the edges up and over the filling, pinching firmly at the top to seal. Place seam-side down on a small square of parchment paper and rotate gently to smooth the shape.

Cover finished manju with a damp towel. Repeat with the remaining dough and filling.

Fill a wok or pot with about 2 inches of water and bring to a rolling boil. Optionally lay damp cheesecloth over the steamer rack.

Place manju in the steamer with about 1 inch of space between each one. Steam over high heat for 10 to 12 minutes.

Remove from the steamer and let cool on a wire rack. While still warm, wrap each manju individually in plastic wrap to retain moisture.