To make the crumble topping, mash together the butter, sugar, flour, and rosemary in a large bowl. The best way to do this is with your hands.
In a large bowl, mix the blackberries, sugar, cornstarch, and lemon juice together. Pour the blackberry filling into a 9×13-inch glass baking pan; even it out with a spatula. Crumble the topping over the blackberry mixture. Bake for 1 hour on a nonstick cookie sheet (to catch any overflow) until the topping is brown and the fruit is bubbly. Serve the crumble with vanilla ice cream. Make sure the crumble is still a little warm so the ice cream will melt.
Edited: October 12th, 2009