Cut the ham into halves.

Mix bread flour, cake flour, baking powder, sugar, and salt in a bowl. Add vegetable oil and water, mixing with a spatula until combined. Knead by hand until smooth, then cover with plastic wrap and let rest for 20 minutes.

Divide the dough into 4 portions. Roll each into a rectangle (8cm x 15cm). Spread 1/4 of the mayonnaise and place 1/4 of the ham on each. Roll tightly and seal the edges.

Place the rolls on a parchment-lined pan with 3 tablespoons of water. Cut diagonal slits every 2cm using kitchen scissors, alternating the direction of each cut.

Cover the pan and steam over low heat for 15 minutes.