Preheat oven to 350°F.
In a 10-inch cast-iron frying pan or a 9x2-inch cakepan over low heat, melt butter or margarine. Stir in brown sugar until blended. Remove from heat. Sprinkle lemon zest over the brown sugar. Sprinkle chopped nuts over the top. Arrange the blackberries in an even layer over the nuts; set aside.
In a large bowl, combine flour, sugar, baking powder, and salt. Add egg, milk, and butter; beat 2 minutes. Add lemon zest, lemon juice, and vanilla extract; beat 2 minutes. Pour over blackberries in frying pan, spreading evenly.
Bake 40 to 50 minutes or until a toothpick inserted in center comes out clean. Remove from oven and cool 5 minutes on a wire rack. Run knife around edge of pan to loosen; cover with a cake plate and invert. Serve warm.
Makes 8 servings.