Place the bread slices in a food processor and pulse until roughly ground with some slightly larger pieces for texture. If you do not have a food processor available, grate the bread slices across the larger holes of a box grater.
Drizzle 2 tablespoons (30 milliliters) olive oil in a wide skillet over medium low heat.
Once heated, add the breadcrumbs and cook, stirring constantly, until golden.
Transfer the breadcrumbs to a bowl, clean out the skillet, and set aside.
Bring a large pot of salted water to a boil.
Reduce heat to a simmer, add the pasta and cook, stirring occasionally, until barely al dente, just tender.
While the pasta is cooking, place the cleaned skillet back on the stove over medium low heat and add the remaining 4 tablespoons (60 milliliters) olive oil.
Once the oil is heated, add the anchovy fillets, garlic, and red chili flakes (if using) and cook, stirring until the anchovies melt into the oil and the garlic is fragrant.
Remove from heat and stir in the parsley.
Once the pasta is barely tender, drain, and reserve 1/4 cup (60 milliliters) of the cooking water.
Place the pan with the oil back over low heat, add the pasta, and toss evenly to coat. Slowly stir in the reserved cooking water if needed to hold everything together.
Season with salt and toss in half of the toasted breadcrumbs.
Adjust seasonings to taste, adding more salt if needed.
Remove from heat and serve immediately with the remaining breadcrumbs, plus a little more parsley and a drizzle of additional olive oil if desired.