In a mixing bowl, take the soft butter and castor sugar. Using an electric hand mixer, beat together for about 2 minutes until the mixture turns light, creamy, and pale.
Add the sweetened condensed milk and vanilla extract. Continue mixing for another 1–2 minutes until everything is well combined and smooth. Scrape down the sides of the bowl with a spatula in between.
Add the all-purpose flour (and pinch of salt if using unsalted butter). Mix until just combined and a soft dough comes together. Do not overmix.
Cover the dough with cling film and refrigerate for 30–60 minutes. This step is essential — it firms up the butter and prevents the cookies from spreading too much while baking.
Preheat your oven to 180°C (350°F). Line a baking tray with parchment paper.
Take the chilled dough out of the fridge. Scoop a spoonful of dough and roll it into a smooth ball between your palms. Place each ball on the prepared baking tray, leaving about 1 inch of space between each cookie.
Press each ball gently with a fork in a crisscross pattern. This flattens the cookies slightly and gives them a beautiful classic finish.
Bake in the preheated oven at 180°C for 12–15 minutes, or until the edges turn golden brown. The centres will look slightly soft — that's exactly what you want.
Remove from the oven and let the cookies cool on the baking tray for 5 minutes — they will firm up as they cool. Then transfer to a cooling rack to cool completely.
Store in an airtight container at room temperature for up to 5–7 days.