Combine the softened butter, mustard, parsley, garlic, lemon juice, paprika, salt, black pepper, cayenne, and red chili flakes in a bowl and mix with a spatula until fully combined.
Set aside to use immediately, or for best results, to allow the flavours to meld, cover in plastic and refrigerate for 2 hours or up to overnight.
Bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally, until tender yet firm to the bite, about 12 minutes. Reserve ¼ cup pasta water, then drain spaghetti and set aside.
Meanwhile, melt the butter in a large saucepan over medium-low heat.
Stir in spaghetti, and add reserved pasta water as needed to thin sauce.