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1
Position an oven rack in the center of the oven and preheat the oven to 325u00b0F/160u00b0C/gas 3. Melt the butter. Using 1 tablespoon of the melted butter, generously butter a 9-in/23-cm by 13-in/33-cm baking dish. Set aside.
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2
To make the streusel topping, combine the remaining melted butter and vanilla in a small bowl. Set aside. Place the oats and brown sugar in the bowl of your KitchenAid(R) Stand Mixer fitted with the flat beater. Attach the KitchenAid(R) Sifter + Scale Attachment to your mixer. Close the ingredient valve. Weigh the flour, spices, and salt in the ingredient hopper, taring the scale between ingredients. Turn the mixer to medium-low speed, open the ingredient valve, and sift the flour mixture into the mixing bowl. While mixing, pour the butter mixture down the side of the bowl and mix just until incorporated, about 1 minute. (Don't overmix or the streusel topping will turn into one big clump rather than a crumbly mixture.) Turn the mixer off, transfer the streusel topping to a bowl, and set aside. Wipe out the mixing bowl and paddle attachment.
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3
To make the fruit filling, in a medium bowl, gently stir together the blackberries, sugar, and salt. Set aside.
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4
To make the coffeecake, spread the nuts in a single layer on a small rimmed baking sheet, and bake until fragrant and starting to color, 5 to 8 minutes. Set aside to cool.
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5
Put the butter in the mixing bowl. Close the ingredient valve. Weigh the sugar in the ingredient hopper. With the mixer on medium-low speed, open the ingredient valve, and sift in the sugar. Cream the butter and sugar on a high speed until light and fluffy, about 2 minutes. Beat in the canola oil, one tablespoon at a time until incorporated. Add the eggs, one at a time, beating well after each addition. Turn off the mixer. Add the sour cream and beat until incorporated, 30 seconds longer. Turn the mixer off and scrape down the sides of the bowl.
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6
Close the ingredient valve. Weigh the flour, salt, baking powder, baking soda, and spices in the ingredient hopper. With the mixer on medium-low speed, open the ingredient valve, and sift in the flour mixture. At the same time pour the buttermilk and vanilla down the side of the mixing bowl. Mix just until incorporated, 30 seconds to 1 minute longer.
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7
Turn the mixer off, remove the bowl from the mixer, and, using a rubber spatula, scrape down the sides of the bowl. Fold in the toasted pecans.
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8
Pour half of the batter into the prepared pan and spread evenly with a small offset spatula. Distribute the fruit over top, spreading it evenly over the batter. Leave a 1/4-inch border so the fruit doesn't burn on the sides of the pan. Top with the remaining batter, spreading it evenly. Crumble the streusel topping and sprinkle it evenly over the top of the batter.
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9
Bake the coffeecake until the streusel is deeply golden brown, and a toothpick inserted into the center comes out clean, 50 to 55 minutes. For even baking, at the midpoint, rotate the pan from front to back. Let the coffeecake cool in the pan on a wire rack for 20 minutes before cutting into squares and serving. The coffeecake can be made 1 day in advance and kept covered at room temperature. (The coffeecake freezes well. Cover tightly and freeze for up 30 days. Thaw at room temperature, cut into squares, and rewarm before serving.)
