When we moved from 0 to 4,700 ft above sea level six years ago, it was a bit of a baking culture shock! Long story short, I spent a LOT of time reading, experimenting, calling foodie friends, swearing, you name it, and after a couple years got back on my game.
As for cookies, I spent about two years tweaking my used-to-be-perfect-at-sea-level-cookie recipe so you don't have to go through the hundreds and batches and delicious calories that I did (hey, someone had to do it. You're welcome).
I use this base all the time as a starter for cookies--add a 1 1/2 cups of your favorite mix-ins and you're ready to roll. Current favorite is Raspberry Dark Chocolate Chunk!! (Just make sure you use dried raspberries instead of fresh, they'll ruin your cookie texture. You can find dried raspberries at Trader Joe's or Wal-Mart).
No-Fail High Altitude Cookie Base
(I usually double the recipe, but I'm not the boss of you)
1 cup butter, softened
3/4 cup white granulated sugar
3/4 cup brown sugar
3/4 tsp. baking soda
1/2 tsp. salt
2 large eggs
1 tablespoon vanilla extract (I prefer vanilla bean paste if you can find it)
2 3/4 cups white flour
1 1/2 cups mix-ins (chocolate chips, dried fruit, nuts, etc)
Preheat oven to 350 degrees F. Cream butter and sugars. Add eggs, one at a time, mixing after each. Add Soda, salt, and vanilla. Add flour and mix well. Add mix-ins. Drop by heaping teaspoon (or use cookie scoop) onto a baking sheet lined with parchment paper or a silpat mat. Bake at 350 for 9-11 minutes, until edges are barely brown. Remove from oven and let sit on cookie sheet for 3-5 minutes to cool and set.
Notes: For Dark Chocolate Chunk Raspberry cookies, use 3/4 c. dried raspberries and 3/4 c. dark chocolate chunks.