In a large bowl, mix flour, sugar, yeast, and salt. Add eggs and warm milk, then mix until a shaggy dough forms.
Add softened butter gradually, mixing until the dough becomes smooth, glossy, and elastic.
Knead for 8–10 minutes. Cover and let rise until doubled, about 1 to 1½ hours.
In a small bowl, mix cream cheese, sugar, egg yolk, vanilla, and flour or cornstarch until smooth and thick.
Divide dough into 8 pieces and roll into smooth balls. Place on a parchment-lined baking sheet.
Press a deep well into the center of each bun.
Spoon vanilla cream into each well, then add a small spoonful of strawberry rhubarb jam on top.
Cover loosely and let rise for 30–45 minutes, until puffy.
Brush the exposed dough with egg wash, avoiding the cream and jam center.
Bake at 350°F (175°C) for 18–22 minutes, until golden and the cream is softly set.
Cool slightly, then dust with powdered sugar before serving.