1. Cut tops off tomatoes. Gently remove seeds and pulp with a small melon baller or measuring spoon. Invert onto a paper towellined baking sheet and let drain.
2. Into the bowl of a food processor, put spreadable cheese and buttermilk. Whirl until smooth. Add cheddar and pulse two or three times.
3. Place cheese mixture into a pastry bag fitted with a medium-size star tip. Pipe cheese mixture into tomatoes.
4. Refrigerate at least 1 hour.