Combine ground beef, sea salt, ancho chili powder by hand, and divide into four equal shaped burger patties.
Transfer patties into a narrow baking dish -- something narrow enough to help the coffee marinade to cover the patties as much as possible.
Pour the cooled strong coffee or cold brew over the burger patties and allow to marinade for 15 minutes. Then flip and marinade the other side for an additional 10 minutes.
While the burgers marinade, preheat the grill. Whether indoor or outdoor, the temperature should be preheated to medium-high (about 375°F) to high (400°F).
Lift the marinated burgers from the coffee and set aside. In a small saucepan, whisk the avocado oil with cacao powder first. The oil helps to blend the cacao powder into the coffee clump-free. Stream the coffee marinade into the oil and cacao, whisking to combine. Set the saucepan on the stove set to medium. Simmer to reduce the marinade until it reaches about 1/4 cup in volume.
Once the coffee-cacao marinade has reduced, begin grilling the burgers. Grill for 4 to 5 minutes on each side, basting with the coffee-cacao reduction frequently in order to create a sticky coffee chocolately glaze on the burgers.
To serve, transfer the burgers to a serving platter and sprinkle with coarse grain sea salt or flaky Maldon.