Step 1

Preheat oven to 350°F and spray a 9 x 5-inch loaf pan with nonstick spray. Line pan with parchment paper, letting excess extend over sides of pan.

Step 2

Whisk together flour, cocoa, baking powder, baking soda, and salt in a medium bowl. Beat butter and sugar in a large mixer bowl at medium speed until light and fluffy; beat in egg and vanilla. Combine buttermilk and sour cream in a small bowl. Reduce speed to low and add half of flour mixture, beating just to combine. Add buttermilk mixture, then finish with remaining flour mixture. Dissolve espresso powder in boiling water and slowly mix in just to combine. Fold in chocolate chips.

Step 3

Pour batter into prepared pan; smooth top with an offset spatula and tap sharply to reduce air bubbles.

Step 4

for 50 to 55 minutes, until a toothpick inserted in the center comes out clean or with a few crumbs attached and cake springs back to the touch. Cool in pan on a wire rack for 15 minutes. While the cake is cooling, top with chocolate chips.

Step 5

Carefully lift cake from pan and cool completely on wire rack. Store leftovers tightly covered in an airtight container at room temperature.