Preheat your oven to the lowest setting it will go. I set my oven to 170°F.
Place parchment paper on a baking sheet and spread coffee grounds on top
3-4 cups coffee grounds
Put in oven until coffee grounds are fully dry and feel slightly crunchy (this took about 3 hours for me). Stir coffee grounds every hour.
Remove from oven and cool grounds
Place a small amount in coffee grinder and grind until very fine
Store espresso powder in an airtight jar or container
Notes
Yield will vary depending on how many grounds you begin with
Actual time will vary depending on the amount of grounds you are using, how wet your grounds are and the temperature of your oven.
I used my fingers to stir the coffee grounds so I could feel how dry they were. When the grounds were still wet, they stuck to my fingertips and it looked like I had been playing in the dirt. When the grounds were dry, they no longer stuck to my fingers.