Preheat the oven to 350°F.

Place the potatoes on a baking tray and bake for about 45 minutes, or until a paring knife slides easily into the center of the potatoes.

While the potatoes are baking, slice the bacon into thin strips. Cook the bacon in a large, heavy-bottomed pot over medium heat until crisp. Remove the bacon with a slotted spoon and set aside, leaving the bacon drippings in the pot.

Add the flour to the bacon drippings and stir to combine. Cook for about 1 minute, without allowing the flour to brown.

Slowly pour in the heated milk while whisking constantly to prevent lumps. Cook over medium heat until the mixture is bubbly and thickened, about 15 minutes, stirring frequently.

When the potatoes are cool enough to handle, cut them open and scoop the pulp from the skins. Add the potato pulp to the milk mixture and mash gently with the back of a wooden spoon, leaving some small chunks of potato for texture.

If using optional vegetables, add them to the soup and cook until heated through and tender.

Add the cooked bacon, reserving some for garnish. Stir in 1 cup of the Cheddar cheese and season to taste with kosher salt and freshly ground black pepper. Continue stirring until the cheese has melted, then remove from the heat.

Ladle the soup into bowls. Top with sour cream, the reserved bacon, remaining Cheddar cheese, and fresh chives.

Serve hot.