Combine the flour, sugar, yeast and salt in the bowl of a stand mixer and whisk together. Add the water and yogurt. Using a rubber spatula, give it a mix to just combine. Place the bowl on the stand mixer and mix with the dough hook on medium speed, scraping down the sides of the bowl and hook as necessary. The dough will be soft and smooth, but sticky, and will pull away from the sides of the bowl when it has mixed long enough, 12 to 15 minutes. Transfer to a large, oiled bowl, cover with plastic, and allow to rise at room temperature until doubled in size, 1 1/2 to 2 hours.
Grease a 9-by-13-inch baking dish with the butter and dust all over with the cornmeal, tapping out any excess.
On a cornmeal-dusted surface, roll out the dough to a rectangle that is roughly 20-by-10-inches and just under 1/2-inch-thick. With a small, offset spatula, spread 1 side with an even layer of strawberry jam, leaving a 1/2-inch border at the top. Repeat with the blueberry jam on the other side. Roll up into a tight log and pinch the edges to seal. With a sharp knife, slice into 12 even rolls. Place alternating rolls in the prepared baking dish and cover with plastic wrap. Refrigerate overnight (anywhere from 8 to 12 hours is fine; try not to exceed 14 to 16 hours) or let rise at room temperature for another until doubled in size, 45 minutes to 1 hour.
Place an oven rack in the middle position. Preheat the oven to 400 degrees F.
When ready to bake, remove the rolls from the refrigerator and let sit at room temperature for an hour (unless they proofed at room temperature). Remove the plastic wrap, brush the tops with the egg wash. Bake until the rolls are a deep golden brown and the top feels hollow when you tap on it, start checking at 25 minutes.
While the rolls are baking, prepare the Glaze (recipe follows).
Remove the rolls from the oven and cool for 10 minutes. Drizzle over the glaze and serve. These can be stored in an airtight container on the counter for a day or so (reheat in the microwave).
Yield:about 1 1/2 cups
Combine the powdered sugar, yogurt and salt in a bowl and whisk until smooth.