Prepare an 8 1/2 x 4 1/2-inch loaf pan or 4 x 10-inch loaf pan by spraying with cooking spray and lining the bottom and sides with parchment paper. Allow the parchment paper to extend beyond the pan on the sides by at least 1 inch, to use as handles to remove the loaf after baking. Set aside.
Preheat the oven to 375F (non-convection/not fan-assisted). *If using a glass loaf pan, reduce the oven temperature to 350F.
Prepare the filling by combining the brown sugar, cinnamon and cocoa powder in a small bowl. Stir to combine. Add the chopped raisins and walnuts to the bowl and stir together. Set aside.
In a medium bowl, combine the flour, baking powder, baking soda and salt. Whisk together well and set aside.
Add the cold butter pieces to the bowl of a stand mixer fitted with the paddle attachment or to a large bowl with an electric mixer. Beat the butter until softened, about 1 minute. Add the white sugar and vanilla and beat together on medium speed for a minute or two. Add the eggs and beat in, scraping down the bowl with a rubber spatula and beating at high speed briefly until the mixture is quite smooth.
With the mixer on low, add about 1/3 of the flour mixture, followed by half of the sour cream, half the remaining flour, the remaining sour cream and finally the remaining flour mixture, beating only until smooth after each addition.
Tip! You will layer the batter into the loaf pan, forming 4 layers of batter and 3 layers of filling in between. I find it helpful to roughly divide the batter into quarters in the bowl as a guide for how much to use for each layer. For the filling, use 1/3 to start, then divide the remaining filling between the next two layers.
Spoon about 1/4 of the batter into the bottom of the prepared loaf pan and use the back of a spoon to spread the batter into an even layer. Sprinkle 1/3 of the filling evenly over the top of the batter. Add another 1/4 of the batter in small spoonfuls scattered over the top of the filling and use the back of a spoon to spread from the batter evenly over the top of the filling. Spread in one direction only from the thickest part of the batter out to avoid picking up bits of the filling in the batter. This process is similar to icing a cake to avoid picking up crumbs. Repeat layering until you have 4 layers of batter and 3 layers of filling, ending with the last quarter of batter and smoothing it level in the pan.
Tip! *Baking times will vary from oven to oven and depending on the colour of your loaf pan. Check regularly during the end part of baking and test with a cake tester when it looks close to done. The tester should come out clean of batter (but may have a bit of the filling attached).
Place the loaf into the preheated oven and bake for 45-50 minutes for an 8 1/2 x 4 1/2-inch metal loaf pan or about 40 minutes for a 4x10-inch loaf pan.
Remove from the oven and allow the loaf to cool in the pan for 10 minutes, then remove the loaf from the pan and place on a cooling rack to cool completely before slicing.
Store the cooled loaf in an airtight container at room temperature for 3-4 days. The loaf will also freeze well for up to 3 months.
Notes
Note 1: To get 1/2 an egg, beat one egg in a small bowl with a fork and use 2 Tablespoons for the 1/2 egg. Reserve or discard the remaining egg. If you'd rather not waste any egg, use 1 whole egg and increase the sour cream by 2 Tablespoons.Note 2: Full-fat sour cream is highly recommended for the best flavour and texture. In a pinch, you could use plain regular or Greek yogurt, but higher fat yogurt will give the best results.Tips!When spreading the batter into the loaf pan in layers, spread the batter like you would ice a cake by spreading from the thickest part of the batter in the centre towards the outside and not making a return trip with the spoon, as that is when you will pick up bits of the filling. That said, don't worry if you get a bit of filling in the batter. It will all be good in the end!Be sure to read the notes above this Recipe Card, for more tips on making this recipe. You'll also find substitution suggestions and step-by-step photos that you might find helpful.