Preheat oven to 350ºF. Coat two 9-inch round pans with cooking spray and line bottoms with parchment paper.
Whisk cocoa with 1-1/2 cups boiling water in a medium bowl until smooth, set aside.
Whisk flour, sugar, baking powder, soda and salt in a large bowl of a until well combined.
Add eggs, canola oil, sour cream and vanilla. Beat with a mixer on medium speed for about 1 minutes until smooth.
Reduce mixer speed to low and add cocoa mixture in a slow stream until just combined. Scrap down sides and give a few turns with a spatula. Batter will be thin.
Divide batter between the pans, tapping on counter to release any bubbles. Bake for 30 to 40 minutes until a toothpick inserted in the center comes out clean.
Cool on racks.
TO ASSEMBLE: Slice each layer in half to make 4 layers. Place 1 layer on a cake plate and spread with 2 TBSP orange marmalade. Repeat with other layers. Cover the whole cake with Chocolate Whipped Cream Frosting (to follow) and drizzle with Salted Caramel (to follow).