Step 1

Preheat oven to 350°F. Spray 3 (9-inch) round cake pans with cooking spray; line bottoms with parchment paper.

Step 2

Prepare the Cake Layers: Whisk together first 7 ingredients in a large bowl; set aside.

Step 3

Place sugar and eggs in bowl of a heavy-duty stand mixer fitted with whisk attachment; beat on medium speed until thick and smooth, 3 minutes. Change to paddle attachment. Add oil, jam, and vanilla extract; beat until combined, 1 minute.

Step 4

Add half of the flour mixture to jam mixture; beat on low speed until just combined, 20 to 45 seconds. Add buttermilk; beat on low speed until just combined. Add remaining flour mixture; beat on low speed until smooth and combined, about 30 seconds. Fold in nuts and raisins; divide evenly among prepared pans.

Step 5

Bake in preheated oven until edges of cake layers are just starting to pull from sides of pans and a wooden pick inserted in center comes out clean, 35 to 40 minutes, rotating pans from top to bottom rack halfway through baking. Press gently on top of layers to flatten any doming; cool in pans 10 minutes. Remove from pans to wire racks; cool completely, about 1 hour.

Step 6

Prepare the Frosting: Place cream cheese and sugar in bowl of a heavy-duty stand mixer fitted with whisk attachment; beat on medium-low speed until combined. Add caramel topping; beat until smooth, 1 to 2 minutes. With mixer running, slowly pour in cream and add salt. Beat on medium speed until thick and smooth, 3 to 4 minutes. (Mixture will look runny at first but will thicken as the cream is whipped.)

Step 7

Transfer about two-thirds of the frosting to a piping bag. Place 1 cake layer on a serving plate. Pipe frosting on top of layer, using a circular motion and starting from the outside and gradually moving to the center. Repeat process with remaining layers. Cover sides with remaining frosting. (Drag the back of a spoon in a side-to-side motion across the frosting for a textured look.) Garnish cake with nuts and blackberries.