Finely grate the zest from both lemons and squeeze their juice; keep the zest and juice separate.

Bring a large pot of well-salted water to a rolling boil over high heat.

Cook the 400 g spaghetti in the boiling water until al dente, following the package timing.

Reserve 240 ml of the pasta cooking water, then drain the spaghetti.

Melt the 120 g butter in a large frying pan over low heat.

Add the 80 g anchovies to the melted butter and stir until they dissolve into the sauce.

Add the drained spaghetti to the anchovy butter and toss to coat.

Add about 120 ml reserved pasta water and toss until the sauce becomes glossy and emulsified.

Remove the pan from the heat and toss in the lemon juice and lemon zest.

Season with black pepper and salt if needed, then serve the pasta immediately.