Blackberry Coffee Cake

Directions

1. 1
   FOR THE TOPPING, stir together the sugar, walnuts, and cinnamon in a small bowl until evenly mixed.
2. 2
   Set aside.
3. 3
   PREHEAT THE OVEN to 350°F Butter a 9- by 13-inch baking dish.
4. 4
   IN A LARGE BOWL, combine the flour, sugar, baking powder, and baking soda and stir to evenly mix.
5. 5
   Add the sour cream, melted butter, and eggs, stirring gently until the batter is just evenly blended.
6. 6
   Avoid overmixing or the cake will lose some of its lightness.
7. 7
   POUR A GENEROUS half of the batter into the prepared baking dish, spreading it out evenly.
8. 8
   Scatter the berries on the batter and pour the remaining batter over them, spreading it evenly (some of the berries may seem too close to the surface, but they'll settle during baking).
9. 9
   Sprinkle the topping evenly over the batter and bake until nicely browned and a toothpick inserted in the center comes out clean, 40 to 45 minutes.
10. 10
    LET THE COFFEE CAKE cool slightly before cutting it into squares to serve.