• In a large bowl, combine the flour, sugar, and salt. Add the cold butter and, using a pastry blender, two knives, or your fingertips, rub the butter into the flour until the mixture resembles coarse crumbs with pea-sized pieces of butter.
• Add the buttermilk and sourdough starter. Gently mix with a spatula until the ingredients are well incorporated. Cover the bowl and let the dough rest on the counter overnight (or for your preferred fermentation time). DO NOT add the baking powder and soda at this point!

Assembly

• Before rolling out the dough, preheat the oven to 190°C (375°F). Grease your cast-iron skillet or baking pan with butter. In a medium bowl, rub together sugar and lemon zest with your fingers until fragrant. Stir in the cornstarch. Add fresh berries on top, but don't stir until the dough is rolled out. Alternatively, keep frozen berries in the freezer while you roll out the dough.
• Turn the dough out onto a clean work surface and gently flatten it into a thick rectangle. The dough will be soft and sticky. Sprinkle the baking powder and baking soda evenly over the top. Gently knead the dough to incorporate the leavening until it feels a little less sticky and more cohesive.
• Generously flour your worktop, rolling pin and the top of the dough. Roll the dough into a rectangle. Try to keep the edges straight to ensure even rolls. Roll it as thin as you feel comfortable with, but not so thin that it tears.
• Gently toss the berries with the lemon sugar to combine, being careful not to crush the berries if they’re fresh. Spread the filling evenly over the dough, leaving a strip along the top edge free of filling. Make sure it reaches all the way to the sides and is spread evenly across the dough.
• Carefully roll the dough into a tight log, pinching the seam closed to seal it. Also, pinch the edges to prevent the filling from spilling out. At this stage, you can gently shape the log to make it more even, removing any thick or uneven areas.
• Cut the log into 12 even sections. To do this, use a bench scraper or knife to visually divide the log into halves, then each half into quarters, and each quarter into three pieces. To cut evenly, use a sharp knife or unflavored dental floss for a clean cut.

Baking

• Place the cut rolls into your prepared baking pan, spacing them apart to allow for expansion as they bake.
• Bake sourdough rolls for 35–40 minutes, or until the tops are golden brown. If using frozen berries, the baking time may be slightly longer. Once baked, allow the rolls to cool to room temperature before glazing.

Glaze

• For the glaze, whisk together powdered sugar and lemon juice in a small bowl until smooth and at your desired consistency. Drizzle the glaze over the cooled rolls in thin ribbons, then let it set before serving.