Start by spreading your espresso grounds on a baking sheet. You’ll want to spread them into a thin layer. At this point, it doesn’t matter how finely ground your coffee is, but you don’t want to use whole beans. If you’re using a puck from your portafilter, make sure you break it all the way up. Don’t worry if your grounds are a little damp: the purpose of baking them in the oven is to dry them out. Depending on how wet they are, you may need to bake them a little longer.
Turn on your oven to a low setting, around 200° F. Put the baking sheet on a center rack, and let it bake for about an hour. You’ll know it’s ready when the grounds are lightly toasted and fully dried.
Take the baking sheet out of the oven and let your grounds cool for a few minutes. Once they’re cool, grind them very finely in a coffee or spice grinder. Do this in batches if you need to.
For the best flavor, you'll want to store your espresso powder in an airtight container. Like any spice, it will lose flavor as it comes into contact with oxygen. The flavor should maintain for at least six months, after which you may want to make a new batch.
The best beans for espresso powder are fresh and darkly roasted. Espresso is not a variety of coffee beans, like Arabica or Robusta, but instead refers to its dark roast, which helps your brewed espresso have bold and smoky flavor.You can use previously brewed or fresh coffee grounds. If you make espresso powder using coffee that hasn’t been brewed, it will have a stronger flavor and higher caffeine content. Use slightly less of this more concentrated powder in your recipes.