Baking at Very High Altitudes

In the US, baking mixes often have high altitude directions for use in cities like Denver, the Mile-High City. However, these directions are only good for altitudes of up to 6,500 feet. What do you do if you own a bakery in La Paz, Bolivia, at an altitude of nearly 12,000 feet? What kind of adjustments need to be made to recipes for baked goods in the Two-Mile-High City?

Well my family has a house which is at about 13,000 feet. AFAIK, the ingredients are the same and baking and cooking generally takes a long time.

Boiling and stove top cooking stuff takes longer because water boils at a lower temperature. There are other considerations with baked goods. The leavening intended for recipes at sea level is wrong, and will basically rise too fast, and too much, and overinflate the cake which then falls. You can easily find some guidelines out there, like these: