1. Preheat the oven to 350° F. Place the walnuts on a rimmed baking sheet and roast for 10-12 minutes until golden brown and fragrant. Set aside to cool. Line a 9 x 5" loaf pan with parchment paper.

2. Melt the 4 tablespoons of butter for the filling mixture, and set aside to cool.

3. In the bowl of a stand mixer, beat the stick of butter and sugar on medium speed until lighter in color and smooth. Scrape down the sides of the bowl with a spatula. Add the eggs, mix to combine, then add the sour cream and vanilla extract. Mix on medium speed until the ingredients are fully incorporated, about 2-3 minutes. Add the flour, baking powder, baking soda, and salt. Mix on low until fully incorporated, about 30 seconds. Don't overmix. The batter will be thick.

4. In a separate bowl, combine the roasted walnuts, brown sugar, cinnamon, and kosher salt with the melted butter, stir to combine.

Assemble the cake

1. Spoon half of the cake batter into the bottom of the prepared loaf pan. Use an offset spatula to level the batter. Sprinkle half of the buttery walnut mixture on top. Then sprinkle half of the halva cubes on top of the nuts. Spoon the remaining batter on top, then level out until smooth and even. Add the remaining nuts on top, then the rest of the halva. Place the cake pan on a rimmed baking sheet, in case of any spillover.

2. Bake at 350° for 45-50 minutes, or until a toothpick comes out with moist crumbs. Allow the cake to cool completely before slicing.