In a large mixing bowl, combine all-purpose flour, granulated sugar, yeast, and salt. Warm milk and butter over low heat until melted, then cool slightly. Add eggs and milk mixture to dry ingredients, stirring until a sticky dough forms.

Turn dough onto lightly floured surface and knead for 8-10 minutes until smooth and elastic. Sprinkle more flour if too sticky.

Shape dough into a ball, place in greased bowl, cover, and let rise in warm place until doubled in size, about 1 hour.

Prepare the custard by mixing custard powder, milk, granulated sugar, and vanilla in a saucepan. Cook over medium heat, stirring until thickened, about 5-7 minutes. Transfer to a bowl to cool slightly.

After dough has risen, punch down gently, roll out into a rectangle about 1/2 inch thick. Spread cooled custard filling evenly over dough. Roll dough tightly into a loaf.

Place shaped loaf seam-side down in greased loaf pan, cover loosely with cloth, and let rise again until doubled, about 30-45 minutes.

Preheat oven to 350°F (175°C). Once dough is ready, bake for 25-30 minutes until golden brown and a skewer comes out clean.

Remove from oven, let cool in pan for 10 minutes, then transfer to wire rack. Dust with powdered sugar if desired and serve warm.