In a bowl, whisk the egg yolks and sugar together until they form a thick paste. Sift in the cake flour and mix until smooth.
In a small saucepan, heat the milk with the vanilla bean seeds and pods over low heat until the edges of the milk bubble. Remove from heat.
Gradually whisk the warm milk into the egg mixture. Return the combined mixture to the saucepan and cook over low heat, whisking constantly, until the custard thickens and large bubbles appear.
Stir in the butter, transfer the custard to a tray, flatten it, and cover it with plastic wrap directly touching the surface. Freeze for 20 minutes, then move it to the fridge.