Instructions

• In a large bowl, whisk together the sifted all-purpose flour (4½ cups), sifted cake flour (¾ cup), instant yeast (2¼ teaspoons), white sugar (4 tablespoons) and salt (½ teaspoon) until evenly combined.
• In a separate bowl, whisk the lukewarm milk (2 cups), the 3 large eggs, vanilla extract (1 teaspoon), vegetable oil (½ cup) and the orange zest from one orange until blended.
• Add the wet mixture to the dry ingredients and mix until a cohesive dough forms. Knead the dough by hand on a lightly floured surface or with a stand mixer fitted with a dough hook until smooth and elastic and just slightly sticky (if it is excessively sticky, dust the work surface very lightly with a small amount of flour).
• Lightly oil a clean bowl, place the kneaded dough in the bowl, turn once to coat the surface with oil, cover with plastic wrap or a clean kitchen towel, and let rise in a warm place until doubled in size, about 1 hour.
• Punch down the risen dough to release excess air, then turn it out onto a lightly floured surface and roll it into a rectangle about 14 inches by 18 inches (approximately 35.5 cm by 45.7 cm), about 1/2 inch (≈1.25 cm) thick.
• Using a pastry cutter, pizza cutter, or knife, cut the rectangle into squares about 2 3/4 inches by 2 3/4 inches (7 cm x 7 cm).
• Make a small indentation in the center of each square with your finger or the back of a spoon and place about 1 teaspoon of the mixed berry jam in the indentation.
• Fold each square in half over the jam and pinch the edges firmly to seal so the jam is enclosed. Place each sealed bun seam-side down on a baking pan that you have prepared by brushing with some of the 1/4 cup butter; brush the sides of adjacent buns with the remaining butter so they do not stick together.
• Cover the arranged buns loosely with plastic wrap or a clean kitchen towel and let them rest in a warm place for about 20 minutes to rise slightly. While they rest, preheat the oven to 350°F (175°C).
• Beat the reserved 1 egg until smooth and, just before baking, brush the tops of the buns with the beaten egg.
• Bake in the preheated oven for 25–30 minutes, until the buns are puffed and golden brown.
• Remove the buns from the oven, let cool slightly on a wire rack, and if desired, sprinkle with powdered sugar before serving.