In a small bowl, whisk the flour, baking soda, and salt together. Set aside.
Using a hand mixer or a stand mixer fitted with the paddle attachment, cream the butter with both types of sugar until smooth. Add the vanilla and mix to combine. Add the dry ingredients and mix on low speed until incorporated.
Line a baking sheet with parchment paper. Using your hands, form the dough into small rounds and use your thumb to press an indentation into the center of each cookie. Arrange on baking sheet and chill in the refrigerator for 1 hour.
Preheat oven to 350°F. Remove cookies from the refrigerator and bake for 10-12 minutes. Cool completely before adding caramel sauce.
To make the sauce, combine the butter and sugar in a saucepan over medium heat. Allow the mixture to boil, then cook for about 1-2 minutes, until it becomes an amber color. Remove from heat and immediately stir in the heavy cream. Cool sauce for 10-15 minutes, then add a teaspoonful of sauce into the indentation on each cookie. Finish with a pinch of sea salt.
Notes
This is a dry and crumbly dough, which is why it's important to use your hands to form into cookies.
Make sure to use cold heavy cream for the sauce.
As an alternative, you can also use store-bought caramel sauce in a pinch.