Preheat oven to 375F and line a 9x13 pan with parchment paper if desired.
First make the blackberry swirl filling. In a small saucepan, combine the blackberries, sugar, and cornstarch. Bring to a boil and cook for just about 30 seconds to 1 minute until slightly thickened. Set aside and allow to cool while you make the rest of the cake.
To make the crumble topping, whisk together the flour, brown sugar, cinnamon and salt. Add the cold butter, cut into small pieces, and work it together with your fingers until the butter has been completely incorporated and the mixture is crumbly. Set aside.
In a large mixing bowl, whisk the melted butter, eggs, milk, sour cream, sugar, brown sugar, and vanilla until smooth.
Add the salt and the baking powder, and whisk in.
Fold in the flour, just until there are no dry pockets of flour left in the batter, don't over-mix.
Pour half of the batter into your prepared baking pan. Top with the blackberry mixture, spreading it out evenly over the batter with an offset spatula. Top with the other half of the batter, and smooth it out a bit to cover the surface as evenly as possible.
Using a butterknife, swirl together the layers of batter and blackberry mixture. Sprinkle the crumble topping evenly over the whole cake.
Bake for about 35-40 minutes, or until a toothpick inserted into the center of the cake comes out without wet batter clinging to it.
Remove from the oven and allow to cool before slicing and serving.
Nutritional information is provided as a courtesy and is an estimate only. This information comes from online calculators. Although The View from Great Island attempts to provide accurate nutritional information, these figures are only estimates.
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