Bring a large Pot of water to a rolling boil over high heat. Add the 1 tbsp sal kosher para el agua de cocción — the water should taste pleasantly salty, like mild seawater. This is your only chance to season the 1 lb fettuccine secado o fresco itself.

Add the 1 lb fettuccine secado o fresco and cook according to package directions until just al dente (usually 9–11 minutes for dried, 2–3 minutes for fresh). It should have a very slight bite — it will finish cooking in the sauce.

Before draining, use a Ladle or Measuring Cups to scoop out at least 1 full cup of the starchy 1 cup agua de cocción de pasta reservada antes de colar — el agua con almidón es el secreto de la salsa and set it aside.

Drain the 1 lb fettuccine secado o fresco, but do NOT rinse it. Rinsing washes away the starch that helps the sauce cling.

Immediately return the empty Pot (or a large Skillet) to the Stovetop over the lowest heat setting. Add the 6 tbsp mantequilla sin sal cortada en cubos, a temperatura ambiente cubes and about ½ cup of the reserved 1 cup agua de cocción de pasta reservada antes de colar — el agua con almidón es el secreto de la salsa. Swirl and stir until the 6 tbsp mantequilla sin sal cortada en cubos, a temperatura ambiente is melted and the mixture looks slightly creamy and emulsified.

Add the drained 1 lb fettuccine secado o fresco to the Pot. Using Tongs, toss the 1 lb fettuccine secado o fresco continuously in the 6 tbsp mantequilla sin sal cortada en cubos, a temperatura ambiente-water mixture for about 1 minute to coat every strand and let the sauce begin to build.

Remove the Pot from the heat entirely. Add the freshly grated 4 oz queso Parmesano recién rallado en un microplane o en el lado más fino de un rallador de caja (aproximadamente 2 cups suelto) in three additions, tossing vigorously with Tongs after each addition. The 4 oz queso Parmesano recién rallado en un microplane o en el lado más fino de un rallador de caja (aproximadamente 2 cups suelto) should melt smoothly into a glossy sauce.

If the sauce looks too thick or the 4 oz queso Parmesano recién rallado en un Microplane o en el lado más fino de un rallador de caja (aproximadamente 2 cups suelto) isn't fully melting, add a splash of reserved 1 cup agua de cocción de pasta reservada antes de colar — el agua con almidón es el secreto de la salsa (1–2 tablespoons at a time) and toss again. Repeat until the sauce is silky, glossy, and coats the 1 lb fettuccine secado o fresco beautifully. You may use up to ½ cup more 1 cup agua de cocción de pasta reservada antes de colar — el agua con almidón es el secreto de la salsa total.

Season with freshly ground 1 tbsp sal kosher para el agua de cocción to taste. Taste for 6 tbsp mantequilla sin sal cortada en cubos, a temperatura ambiente and adjust if needed (the 4 oz queso Parmesano recién rallado en un Microplane o en el lado más fino de un rallador de caja (aproximadamente 2 cups suelto) is quite salty, so you may not need any).

Divide immediately into warm bowls. Garnish with extra 4 oz queso Parmesano recién rallado en un Microplane o en el lado más fino de un rallador de caja (aproximadamente 2 cups suelto) and a sprinkle of fresh 1 tbsp perejil fresco picado finamente if desired. Serve right away — Alfredo waits for no one.