High Altitude Cake Recipe Adjustments?

I need some help with what adjustments. Both layers I made fell in the middle during baking. I didn't open the oven prematurely and am assuming I need to modify ingredients and perhaps the temp and time (350 for 30-35 minutes). I live at 9600ft elevation.

I'd love to hear any suggestions. Thx!

Sandra

INGREDIENTS

• 2 cups all-purpose flour
• 2 cups sugar
• ¾ cup unsweetened cocoa powder
• 2 teaspoons baking powder
• 1½ teaspoons baking soda
• 1 teaspoon salt
• 1 teaspoon espresso powder
• 1 cup milk
• ½ cup vegetable oil
• 2 eggs
• 2 teaspoons vanilla extract
• 1 cup boiling water