Preheat grill to medium-high. Combine coffee, ancho chile, cumin, sugar, 1/2 teaspoon of salt, and pepper in a small bowl. Scatter this rub all over the steak, patting it in gently. Let stand for at least 15 minutes.
Meanwhile, mash avocado in a bowl with the yogurt and remaining 1/4 teaspoon salt until creamy and spreadable.
Grill steak until darkly crusted, 5 to 7 minutes per side (140ºF internal temperature for medium-rare or 155ºF for medium). Remove to a cutting board and let rest 5 minutes. Thinly slice steak across the grain.
Meanwhile, coat bread all over with cooking spray and grill until lightly toasted, about 1 minute per side. Spread avocado mixture on all slices of toast. Top half of slices with steak, tomato slices, lettuce, and the remaining toast. Compress slightly and cut in half on the diagonal.
Tip: The avocado spread on this sandwich makes a great all-purpose spread or dip for tortilla chips.