For the crust: Preheat the oven to 350 degrees F. Butter a 9-inch springform pan and line with parchment paper. Set aside.
Combine the flour, polenta, powdered sugar, cinnamon, baking powder and salt in a medium bowl and stir together with a rubber spatula. Pour in the butter and stir until combined. Add three-quarters of the dough to the bottom of the prepared pan and press evenly into the bottom and 1/2-inch up the sides of the pan. Reserve the remainder of the dough for the top. Bake the crust just to set, 12 to 15 minutes.
For the filling: Combine the cherries, cranberries, muscovado sugar, flour, orange zest and salt in a medium bowl, and stir to combine and coat evenly. Pour the filling into the crust and spread evenly, pressing gently to nestle into the crust. Drop dollops of the remaining dough randomly over the top of the cobbler. Bake until the filling is bubbly and crust and topping are golden brown, about 45 minutes. Allow to cool completely in the pan before releasing the sides and slicing into wedges. Serve with whipped cream or vanilla ice cream if desired.