Prepare Tangzhong by mixing equal parts of flour and water in a saucepan over medium heat for 2-3 minutes until thickened.

Mix dry ingredients in a stand mixer: flour, sugar, salt, yeast, and milk powder on low speed for 1 minute.

Combine wet ingredients: whisk eggs and cold milk, fold in cooled tangzhong, and add to dry ingredients.

Knead dough on low speed for 15 minutes until smooth and elastic, scraping sides occasionally.

Rest dough covered for 10-15 minutes to relax gluten.

Knead rested dough for an additional 5 minutes until elastic and smooth.

Incorporate softened butter gradually while mixing until fully absorbed.

Let dough rise in a greased bowl covered for about 1 hour until doubled.

After rising, punch down dough, cover, and refrigerate for at least 2 hours or overnight.

Shape chilled dough into 10 pieces, roll into balls, and place on baking trays to rise until tripled.

Preheat oven to 190°C (375°F) and bake for 12 minutes until golden brown.

Make Chantilly Cream by whipping heavy cream, mascarpone, and powdered sugar until stiff peaks form.

Fill buns by slicing in half, adding jam, and piping Chantilly cream on top before serving.