Preheat oven to 325°F (170°C).

In the bowl of stand mixer fitted with the paddle attachment, beat butter and granulated 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, salt, and baking powder. With mixer on low speed, gradually 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. Transfer one-third of batter (about 1¾ cups or 421 grams) to another medium bowl; fold in melted chocolate and cocoa until combined. Fold orange zest into remaining batter.

Spread one-third of orange batter (about 1 cup or 281 grams) in bottom of a Baker’s Secret 9x5-inch loaf pan. Spoon orange and chocolate batters by heaping tablespoonfuls (about 18 grams each) in a checkerboard pattern in pan (3 scoops wide along short side). Tap pan 2 to 3 times to help level batter and fill pan. Create another layer in a checkerboard pattern, being sure to place orange 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 figure eight pattern. Tap pan again 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 40 minutes to 1 hour 45 minutes, 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. Garnish with confectioners’ sugar, if desired.