Sea level baking time per recipe: 23 to 30 minutes.
Baking time at 9,000 feet: 35 minutes.

1. Adjust oven rack to center position and heat oven to 350 degrees. Line bottoms of cake pans with parchment paper.

2. Mix cocoa powder and instant espresso in small bowl; add boiling water and mix until smooth. Cool to room temperature, then stir in vanilla and sour cream.

3. Beat butter in bowl of electric mixer set at medium-high speed until smooth and shiny, about 30 seconds. Gradually sprinkle in sugar; beat until mixture is fluffy and almost white, 3 to 5 minutes. Add eggs one at a time, beating 1 full minute after each addition.

4. Whisk flour, baking soda and salt in medium bowl. With mixer on lowest speed, add about 1/3 of dry ingredients to batter, followed immediately by 1/3 of cocoa mixture; mix until ingredients are almost incorporated into batter. Repeat process twice more. When batter appears blended, stop mixer and scrape down sides with rubber spatula. Return mixer to low speed; beat until batter looks satiny, about 15 seconds longer.

5. Divide batter evenly between pans. With rubber spatula, run batter to sides of pan sides and smooth top. Bake cakes ntil they feel firm in center when lightly pressed and skewer comes out clean or with just a crumb or two adhering, 23 to 30 minutes. Transfer pans to wire racks; cool for 20 minutes. Run knife around perimeter of each pan, invert cakes onto racks, and peel off paper liners. Re-invert cakes onto additional racks; cool completely before frosting.