Make the biscuits: Heat oven to 400°. Whisk flour, sugar, baking powder, and salt in a bowl. Using the coarse side of a box grater, grate frozen butter into flour mixture; mix to combine. Add buttermilk and, using your hands, gently mix ingredients until a soft dough forms. Transfer dough to a lightly floured surface; pat into a 7" x 9" rectangle, about 2" thick. Using a 4" round cutter, cut out 4 biscuits, reusing scraps as needed. Place biscuits on a parchment paper—lined baking sheet. Bake until golden, 18–20 minutes. Stir melted butter and honey in a bowl; brush over hot biscuits. Return biscuits to oven and cook until golden, 5 minutes more.
Prepare the pancetta: Arrange pancetta slices in a single layer on 2 baking sheets; bake until crisp, 5–7 minutes. Pour fat from pancetta into a 12" skillet; set aside.
Make the braised collard greens: Bring a large pot of salted water to a boil. Cook collards until tender, 4–6 minutes; drain. Heat skillet with reserved pancetta fat over medium-high; cook onion until soft, 4–6 minutes. Add cooked collards, the stock, salt, and pepper; simmer until stock has evaporated, 10–12 minutes.
Make the espresso aïoli: Whisk mustard, garlic, 1 egg yolk, and salt in a bowl; while whisking, slowly drizzle in oil, and then espresso, until aïoli is emulsified.
Make the marbleized eggs: Melt 1 tbsp. butter in an 8" nonstick skillet over medium; crack 2 eggs into skillet and break the yolks. Using the tip of a heatproof rubber spatula, gently swirl yolks into whites, making a flat omelette and taking care not to scramble the eggs. Cook until bottom is set, about 3 minutes. Add 1 tbsp. chives and salt; fold omelette into quarters, transfer to a plate, and keep warm. Repeat with remaining ingredients to make 4 omelettes.
Assemble sandwiches: Slice biscuits in half; spread halves with aïoli and divide collards, pancetta, and omelettes between biscuits.