Preheat the oven to 200 F. Position racks in the upper and lower thirds of the oven.

In a clean, dry bowl place the egg whites with the cream of tartar. Using a whip attachment, beat at medium high speed with a stand mixer or high speed with a hand mixer until the egg whites are creamy white and hold a soft peak. Continue to beat, adding a little sugar at a time, until you've used all the sugar. This should take 1 1/2 - 2 minutes, until the whites are very stiff.

Sprinkle the salt, dissolved espresso and espresso powder over the stiff egg whites. Gently fold them into the egg whites, making sure the espresso and espresso powder are evenly distributed.

Fill a pastry bag with the batter and pipe approximately 2 inch cookies onto a parchment or silicone lined cookie sheet.

Bake for about 2 hours, rotating the pans front to back and top to bottom halfway through. Remove a test cookie and let it cool completely--warm meringue cookies are apparently always soft. You are testing for crispiness. At such a low temperature, the remaining cookies will not burn while they wait for your test. If the cookie is not completely crisp (it should not be chewy or stick in your teeth at all), continue baking and test at 15-20 minute intervals (note that if it is extremely humid, it may take quite a bit longer--I finally left mine in the oven overnight).

When the cookies are crisp, turn the heat off and let them cool in the oven. If you have a new oven, like me, it may take forever for them to cool; I took them out after about 1 1/2 hours, when the pans were still warm and let them cool on my counter.

Cool completely before storing but do store in an airtight container immediately after cooling. I won't claim they store indefinitely (Medrich says several weeks) but they definitely store well.