Preheat oven to 350°F (180°C). Spray an 8½x4½-inch loaf pan with baking spray with flour. Line pan with parchment paper, letting excess extend over sides of pan.
In the bowl of a stand mixer fitted with the paddle attachment, beat butter and sugar at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add eggs, one at a time, beating well after each addition. Beat in vanilla.
In a medium bowl, whisk together flour, baking powder, and salt. With mixer on low speed, add flour mixture to butter mixture in three additions alternately with sour cream, beginning and ending with flour mixture, beating just until combined after each addition.
In another medium bowl, stir together one-third of batter (about 1⅓ cups or 325 grams), melted chocolate, and cocoa until combined.
Spread one-third of vanilla batter (about 1⁄2 cup or 216 grams) in bottom of prepared pan. Spoon vanilla and chocolate batters by heaping tablespoonfuls (about 15 grams each) in a checkerboard pattern into pan (3 scoops wide along short side). Tap pan on a kitchen towel-lined counter 2 to 3 times to help level batter and fill pan. Create another layer of batters in a checkerboard pattern, placing vanilla on top of chocolate from first layer and vice versa. Tap pan again. Repeat with any remaining batter. Using a knife, swirl batters together in a zigzag pattern through length and width of pan. Tap pan to help level batter and fill in any gaps.
Bake until a wooden pick inserted in center comes out clean or an instant-read thermometer inserted in center registers 200°F (93°C), 1 hour and 20 minutes to 1½ hours, covering with foil to prevent excess browning, if necessary. Let cool in pan for 10 minutes. Remove from pan, and let cool completely on a wire rack.
Wrap cooled cake in plastic wrap, and refrigerate for 1 hour.
Unwrap cake, and return to wire rack. Pour Hazelnut Crunch Ganache all over top and sides of cake, spreading with an offset spatula to fully cover. Refrigerate until set, about 10 minutes, or let stand at room temperature for 25 minutes. Store in an airtight container for up to 3 days.