Combine brown sugar, cumin, chili powder, paprika, salt and pepper in a small bowl. Rub all over pork tenderloin and let sit, uncovered, for 30 minutes or chill up to 8 hours.
Preheat grill to medium heat. Clean and oil grates. Grill pork, turning every 5 minutes, until lightly charred all over and an instant-read thermometer inserted into the center of the loin registers 145°F, about 25 minutes. Let rest 10 minutes then slice very thinly.
While pork cooks, rub peppers all over with olive oil and place directly on the grill. Cook for 10-15 minutes, turning a few times, until fully charred and softened.
Transfer to a sheet of foil and seal up in a packet to steam. Let rest 10 minutes. Rub off the charred skin, cut in half and scrape out seeds. Slice peppers into strips and set aside.
To assemble sliders, place few slices of avocado onto a bottom bun. Use a fork to lightly smash the avocado into the bread. Layer with thinly sliced pork, a few slices of jalapeños and fresh cilantro leaves. Generously spread the top bun with 1 Tablespoon Challenge Spreadable Butter with Olive Oil and Sea Salt and place on top of the sandwich. Repeat with remaining sliders and serve.