Pass the flour, baking powder, sugar, and salt through a sifter into a large mixing bowl. The dry mixture should look evenly blended, without compacted flour lumps.

Blend the beaten egg with the milk and melted butter in a medium bowl using a whisk. Look for an evenly colored liquid without streaks of egg.

Whisk the wet mixture into the sifted ingredients until combined. The batter should pour smoothly rather than fall in heavy clumps.

Transfer the mixture to a measuring jug or spouted measuring cup. A controlled stream will help you reach the mold’s narrow details.

Warm the taiyaki pan and brush its cooking surfaces with the neutral-flavored oil. A thin coating should reach the fins, tail, and edges without pooling.

With the pan over medium-low heat, pour batter into each lower cavity until about 60% full. Leave visible room for both the custard and the covering batter.

Place pastry cream along the center of each fish, then pour enough batter over it to cover. Keep the filling away from the perimeter so batter can seal around it.

Shut the mold, flip it immediately, cook for 2 to 2.5 minutes, then turn it back and cook the other side for another 2 to 2.5 minutes before transferring the cakes to a wire rack. Check for a golden shell and cooked batter, with crisp edges that release from the mold.

Set a medium saucepan containing the milk over medium heat to warm. The milk needs to be hot for gradually warming the yolk mixture.

Beat the yolks and granulated sugar together in a bowl with a whisk. Continue until the mixture becomes paler and airy.

Work the cornstarch into the sweetened yolks with vigorous whisking. The mixture should look smooth, with no dry pockets.

Measure 1/4 cup of the hot milk and gradually whisk it into the yolk mixture. This loosens the thick mixture while gently raising its temperature.

Gradually incorporate the rest of the hot milk while continuing to whisk. The mixture should become fluid and evenly blended.

Return the mixture to the saucepan through a strainer, then whisk continuously over medium-high heat until thick with slow bubbles breaking through. The strainer catches cooked egg bits, while the finished cream should leave visible whisk trails.

Take the pan off the burner, blend in the butter and vanilla, and allow the cream to cool slightly. The butter should disappear into a glossy, smooth custard.

Lay plastic wrap directly against the custard’s surface. Contact with the cream prevents an exposed surface from drying into a skin.

Refrigerate for at least 2 hours, keeping the custard chilled until 1 hour before use. The filling should become firm enough to spoon neatly onto the batter.