Per 100g of pasta:
35g butter
40g/40ml of pasta water
10-15g Parmigiano Reggiano. (if possible: minimum 24-month old. 30 or 36 is better).

While waiting for your pasta water to boil, put a saute pan over medium to medium high heat. Heat the butter until melted and slightly frothy.

Set aside.

When the pasta is cooked, return the sauté pan to medium-high heat.

Add drained pasta to your pan.

Add about 40ml pasta water to the pan, and vigorously swirl around/mix to emulsify.

The sauce will likely start out too watery. Keep stirring over medium high heat until some water evaporates and the consistency just starts to become creamy.

Remove pan from the heat. Add parm and mix until incorporated.

If you do this over heat, the cheese will melt and separate from the sauce. It will still taste good, but it will be stringy and the texture will be somewhat ruined.