Preheat oven to 375 °F and prepare lined baking sheets with parchment paper.
Melt butter and brown sugar in a saucepan over medium heat. Stir often until mixture is smooth.
Remove from heat and stir in oats, flour, salt and vanilla extract. Add in egg when mixture has cooled down so it doesn't cook into a scrambled egg.
Place a tablespoon of cookie mixture on lined baking sheet. Leave about 2 inches distance from each cookie. They will spread and fall flat. Bake for about 10 minutes or until edges turn golden brown.
Remove baking sheet from oven and don't touch cookies. They are too soft and warm to move. After 15 minutes you can use a spatula and place cookies on wire rack to complete the cooling process.