In a large mixing bowl, combine flour, sugar, instant yeast, and salt. In a separate bowl, whisk together warm milk and melted butter until slightly cooled, then add to the dry ingredients and mix until a shaggy dough forms. Knead the dough on a floured surface for about 8-10 minutes until smooth and elastic. Place it in a greased bowl and cover with a kitchen towel. Let it rise in a warm place until doubled in size, about 1 hour.

While the dough rises, prepare the custard filling. In a medium saucepan, whisk together milk, sugar, egg yolks, vanilla extract, and cornstarch until well combined. Heat over medium-low, stirring constantly, until the mixture thickens and bubbles, about 5-7 minutes. Once thickened, remove from heat and transfer to a bowl. Cover with plastic wrap and allow it to cool completely before using in your sugar raspberry danish buns.

Once the dough has risen, punch it down and turn it out onto a lightly floured surface. Divide the dough into equal pieces and shape each into a ball. Flatten each ball into a circle, approximately 4-5 inches in diameter. Add a spoonful of cooled custard and a spoonful of raspberry jam in the center of each circle, then fold the edges over the filling to create a sealed bun. Place the shaped buns seam side down on a baking sheet lined with parchment paper.

Cover the shaped buns with a clean kitchen towel and allow them to rise for another 30 minutes or until slightly puffed.

Preheat your oven to 200°C (390°F). Once the buns have risen, place them in the oven and bake for 15-18 minutes until they are golden brown and fragrant.

Let the baked buns cool on the baking sheet for about 5 minutes before transferring them to a wire rack. If desired, whisk together powdered sugar and a splash of milk to create a glaze, then drizzle it over the warm sugar raspberry danish buns.