Preheat the oven to 375°F. Line a baking sheet with parchment paper and set aside.
In a medium skillet over medium-low heat, melt 2 tablespoons of the butter. Add the leeks and brown sugar and stir to combine. Reduce the heat to low and cook, stirring only very occasionally, until deeply brown and caramelized (see Note). Set aside.
Meanwhile, in a large bowl, whisk together the flour, granulated sugar, baking powder, salt and pepper. Set aside.
In a medium skillet over medium-high heat (ideally, the one you caramelized the leeks in), cook the pancetta until crispy but still chewy, reserving the fat. Add the remaining 2 tablespoons butter to the pan and stir to melt.
Add the leeks and cheese to the flour mixture and stir until evenly coated (to reduce clumping), then add the pancetta along with the fat. Pour in the buttermilk and vinegar and mix until combined.
Using a 2-ounce scoop, portion the dough onto the prepared baking sheet and bake for 20 to 25 minutes, until golden brown on top and deep golden brown and crispy on the bottom.