Blackberry Coffee Cake

I needed to make something for a potluck breakfast that I had organized at work to welcome a new employee.  I had a box of blackberries left in my fridge from an impulsive "ooo, blackberries are on sale, I'm sure I can find something to do with them," so off I went in search of a recipe.  I don't have any large tupperware dishes, so making some kind of coffee cake seemed like a good bet to me.  While searching I stumbled across a blackberry lemon coffee cake recipe.  As I read through the ingredient list, sour cream caught my eye.  Perfect!  I had a tub of leftover sour cream from the potluck dinner a few nights before.  As I haven't yet developed a taste for sour cream as a topping, baking with it seemed like the best way to prevent it from languishing in my fridge past the expiration date.  I managed to run out of lemons over the weekend, but a friend had very conveniently left a lime at my house, so I decided it would become a blackberry lime coffee cake, fancy no?  Finally, my glass pan with convenient plastic lid for transporting is slightly smaller than 9x13, so the fact that I had a 170g container of blackberries (I think it's about 1 cup) rather than 1 1/2 cups didn't seem to matter.  There was plenty of blackberry goodness.  I was a little skeptical after making that batter about this cake because it seemed absurdly thick, but it baked up to be moist and fairly light.  I was also a little concerned when I pulled it out of the oven, that there was extra butter on the top just sitting there melted (I was too lazy to trim/measure and just tossed the final remnants of butter than I had in the microwave to make the topping and I think it was a bit more than 1/4 cup), but I mopped a little up with a paper towel and cooked it a little longer and there were no complaints.

Blackberry Coffee Cake (only slightly adapted from Week of Menus)

Ingredients
1 1/2 cups all-purpose flour
1 1/2 teaspoons double-acting baking powder
pinch of salt
1 cup sour cream
1 teaspoon baking soda
1/2 cup unsalted butter, softened
1 cup granulated sugar
2 large eggs
1 teaspoon vanilla extract
Zest of one lime
170g fresh blackberries
Topping:
1/4 cup butter melted
1/2 cup all-purpose flour
1/4 cup sugar

1. Butter a 9X13 pan.  Preheat oven to 350. 
2. In a bowl whisk together the flour, the baking powder, and a pinch of salt.
3. In another bowl stir together the sour cream and the baking soda.  (It should bubble up a bit in the time it takes you to do everything else.)
4. In a large bowl cream together the butter and 1 cup of the granulated sugar until the mixture is light and fluffy, add the eggs, 1 at a time, beating well after each addition, and beat in the vanilla and lemon zest.
5. Stir in the flour mixture alternately with the sour cream mixture and stir the batter until it is combined well. (1/3 flour, 1/2 sour cream, 1/3 flour, 1/2 sour cream, 1/3 flour until it's all mixed together)
6. Pour (more like spoon) the batter into a buttered 9 X 13 cake pan. Sprinkle blackberries evenly on top.
7. In a small bowl stir together melted butter, flour and sugar until it is sort of dough like and stuck together.
8. Crumble topping mixture all over cake.
9. Bake the cake in the middle of a preheated 350°F. oven for 35 to 45 minutes, or until a tester comes out clean. Transfer it to a rack, and let it cool.