Alix said:
We are at 2192 feet above sea level here. Recipe follows:
1 cup softened butter (unsalted)
2 cups sugar
3 eggs
2 tsp vanilla
2 1/3 cups sifted flour
2/3 cup dutch process cocoa
1 tsp baking powder
1/2 tsp salt
1 cup sour cream
3/4 mini chocolate chips
Beat butter at medium speed 1 minute or until smooth and creamy. Add sugar; beat 5 minutes or until light and fluffy. Add eggs one at a time, beating well after each addition. Beat in vanilla. Scrape down sides.
In large bowl, sift together flour, cocoa, baking soda, baking powder and salt. At low speed add flour in 3 portions, alternating with sour cream and ending with flour mixture. Beat 5 seconds after each addition. Add chocolate chips; finish stirring batter with spatula until blended. Bake at 350 for 45 to 50 minutes or til skewer comes out clean.
Alix,
You are right at the lower limit for where adjustments are needed. Generally speaking, no adjustments are needed below 2,000 feet.
At your altitude, the only thing you need to change is to reduce the amount of leavening you're using. Cut your baking powder by 15%. I noticed you didn't list any baking soda, but you mention it in the directions. That also has to be reduced by 15%.
What the reduction in leavening will do is it will cut down on the amount of CO2 that's generated in the batter as the cake bakes. This will keep the gas cells in the batter smaller. The smaller gas cells makes for better cake structure. If you have too much leavening, the gas cells will be larger and the walls between the cells will be thin and weak which causes the cake to fall.
Hope this helps you understand what's going on.
BTW, if you were at higher altitude, you'd have to increase the amount of eggs and flour along with reducing the leavening. Those adjustments start at around 2,500 feet.
heehee! Guess I was typing too fast. 1 tsp of baking soda too...LOL. 
