Bring a large pot of water to a boil and season generously with salt. Cook the fettuccine according to the package directions until al dente. Before draining, reserve 1/2 cup (120 mL) of pasta water. Drain the pasta and set it aside.

Pat the chicken pieces dry and season with 1 teaspoon kosher salt and 1/2 teaspoon black pepper. Toss to coat evenly.

Heat the olive oil in a 12-inch deep skillet over medium-high heat. Add the chicken in a single layer and cook for 5–7 minutes, turning occasionally, until browned and the thickest pieces reach 165°F (74°C). Transfer the chicken to a plate.

Reduce the heat to medium and add the butter to the same skillet. Once melted, add the sliced jalapeños and cook for 3–4 minutes, stirring occasionally, until softened and lightly browned around the edges.

Stir in the minced garlic, smoked paprika, and optional cayenne pepper. Cook for about 30 seconds, just until fragrant. Avoid browning the garlic deeply.

Pour in the heavy cream and chicken broth, stirring to loosen the browned bits from the bottom of the skillet. Bring to a gentle simmer over medium-low heat and cook for 3–4 minutes. The sauce should begin to coat the back of a spoon but remain pourable.

Reduce the heat to low: Gradually stir in the Parmesan until melted and smooth. Add 1/4 cup (60 mL) of the reserved pasta water and stir until incorporated.

Return the cooked chicken and drained fettuccine to the skillet. Toss with tongs until the pasta is evenly coated and the chicken is distributed throughout.

Add the remaining 1/4 cup (60 mL) pasta water a little at a time as needed. The finished sauce should be creamy and cling to the noodles rather than pool heavily at the bottom of the skillet. If it looks too thin, simmer gently for 1–2 minutes.

Remove the skillet from the heat and stir in the lemon juice. Taste and adjust the seasoning if needed.

Let the pasta rest for 5 minutes. The sauce will thicken slightly as it cools. Sprinkle with optional parsley and serve warm.