Heat oven to 400 degrees, with rack set in the top third.

Halve each squash lengthwise. Leaving a 1/4-inch border, scoop out halves with a small spoon to make boats. Set skins aside.

Roughly chop squash and reserve. Place squash skin halves skin side down, in a shallow baking dish. Season with salt and pepper, and set aside.

In a large skillet, heat oil over medium-high. Add reserved chopped squash, bell pepper, scallion whites, and garlic; season with salt and pepper. Cook, stirring occasionally, until bell pepper begins to soften, 3 to 4 minutes.

Add tomato paste and chili powder; cook, stirring, until fragrant, 1 minute. Add buffalo, cook until no longer pink, breaking up meat with a spoon, 2 to 3 minutes. Remove from heat.

Season with salt and pepper.

Dividing evenly, spoon mixture into squash halves. Cover and bake until squash is tender, 30 to 35 minutes.  

Garnish with scallion greens.