Creating Blackberry Raspberry Sweet Rolls is a straightforward process. Follow these steps for delicious results:

1. Prepare the Dough: In a large mixing bowl, combine flour, sugar, salt, and instant yeast. Mix well.
2. Mix Wet Ingredients: In another bowl, whisk together the warm milk, melted butter, and eggs until combined.
3. Combine Mixtures: Gradually add the wet ingredients to the dry ingredients, mixing until a soft dough forms.
4. Knead the Dough: Transfer the dough to a floured surface and knead for about 5-7 minutes until smooth and elastic.
5. First Rise: Place the dough in a greased bowl, cover it with a damp cloth, and let it rise in a warm place for about 1 hour, or until doubled in size.
6. Prepare the Filling: In a bowl, mix the mashed blackberries, raspberries, brown sugar, cornstarch, and vanilla extract. Set aside.
7. Roll Out Dough: Once the dough has risen, punch it down and roll it out on a floured surface into a rectangle about 1/4 inch thick.
8. Spread the Filling: Evenly spread the berry mixture over the dough, leaving a small border around the edges.
9. Roll the Dough: Starting from one long side, carefully roll the dough into a log shape. Pinch the seams to seal.
10. Slice Rolls: Cut the rolled dough into 12 equal pieces and place them in a greased baking dish.
11. Second Rise: Cover the rolls with a cloth and let them rise again for about 30 minutes.
12. Preheat Oven: While the rolls are rising, preheat your oven to 350°F (175°C).
13. Bake: Once the rolls have risen, bake in the preheated oven for 25-30 minutes, or until golden brown.
14. Cool: Remove from the oven and let the rolls cool in the pan for about 10-15 minutes.
15. Prepare the Glaze: In a small bowl, mix together powdered sugar, milk, and vanilla extract until smooth.
16. Glaze the Rolls: Drizzle the glaze over the warm rolls before serving.

These steps will guide you through creating these delightful Blackberry Raspberry Sweet Rolls, ensuring a delicious outcome every time.