Bring a large pot of well salted water to the boil and cook the pasta, undercooking by 1 minute from the package instructions so it can finish cooking in the sauce.
Reserve a cup of pasta water before draining.
While the pasta cooks, heat the olive oil in a wide pan over medium-low heat. Add the anchovies and cook, stirring, until they melt completely into the oil.
Add the garlic (and red pepper flakes if using) and cook for 30 seconds, just until fragrant.
Add the white wine and increase the heat slightly if needed so it is at a gentle simmer then simmer for 3 - 4 minutes until reduced by about half.
Add the butter pieces in, one at a time, whisking and letting each piece melt in between each addition.
The sauce should have thickened slightly at the end of this process.
Add the cooked pasta with ¼ cup pasta water and the pepper and toss well. Let it bubble away very gently for 1 - 2 minutes until the sauce coats the pasta well. Add another splash of pasta water if you need to loosen.
Taste for seasoning and add salt to taste (it shouldn't need much, if any salt).
Remove from the heat, add the chopped parsley (and capers if using), toss again, and serve.