Start boiling a pot of water for the pasta, add just a tiny pinch of salt (or none at all) since the anchovies are salty.
While the water heats, melt about 2-3 tbsp of the butter in a large pan over low-medium heat.
Add the 4 anchovies and let them gently melt/disintegrate into the butter, stirring occasionally Keep the heat low so it doesn’t burn.
Add the zest of half the lime to the butter-anchovy mix for that fresh aroma.
Cook the tonnarelli until al dente (follow package timing, but taste early).
Reserve a good cup of starchy pasta water before draining.
Transfer the pasta directly to the pan with tongs.
Turn the heat to high and toss everything together, adding splashes of hot pasta water to create a glossy, emulsified sauce.
If it looks dry, add more water; if the sauce starts to separate, toss in a bit more cold butter to bring it back together.
Let it finish cooking in the pan for 1-2 minutes, tossing vigorously.
Turn off the heat, add the zest of the remaining half lime (and a squeeze of juice if you want extra brightness), plus a generous grind of black pepper.
Mix well, plate it up, then finish with the extra 3 anchovies on top, more fresh lime zest, cracked black pepper, e buon appetito!
