Heat oven to 400°F. Lay bacon on a rimmed sheet pan and bake for 15 to 20 minutes until crisp, then drain on paper towels and cool for 10 minutes.
Cut tomatoes into 24 thin usable rounds, reserving the rounded ends for another use. Lay rounds on paper towels and blot their top surfaces.
Chop the cooled bacon finely and mix with mayonnaise, Dijon and pepper. Cut or tear romaine into 24 small pieces that fit the bread.
Spread the bacon mayonnaise thinly over one face of all 48 bread squares. Put a lettuce piece and one tomato round on 24 squares.
Close with the remaining bread, spread sides inward, to make 24 complete mini BLTs. Refrigerate covered until serving within 4 hours; allow two sandwiches per portion.