Preheat oven: Preheat your oven to 350°F (175°C). Grease and flour a 9×5-inch loaf pan, or line with parchment paper for easy removal.
Melt the chocolate: In a heatproof bowl, melt the chopped chocolate with the coffee (or water) over a saucepan of simmering water (double boiler method), or microwave in 20-second intervals, stirring in between until smooth. Set aside to cool slightly.
Mix dry ingredients: In a medium bowl, sift together the cake flour, baking powder, and salt. Set aside.
Cream butter and sugar: In a large mixing bowl, beat the butter and sugar together until light and fluffy (about 3–4 minutes).
Add eggs and vanilla: Beat in the eggs one at a time, then mix in the vanilla extract.
Alternate dry ingredients and sour cream: Add the flour mixture in three parts, alternating with the sour cream in two parts (begin and end with flour). Mix just until combined—do not overmix.
Marble the batter: Divide the batter in half. Stir the melted chocolate into one half of the batter until fully incorporated.
Layer and swirl: Spoon alternating dollops of vanilla and chocolate batters into the prepared loaf pan. Use a knife or skewer to gently swirl the batters together for a marbled effect. Do not over-swirl.
Bake: Bake for 45–55 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Cool: Let the bread cool in the pan for 10–15 minutes, then remove and cool completely on a wire rack.
