With a hand mixer or a stand mixer fitted with the paddle attachment, beat together the butter and brown sugar until the mixture is light and fluffy (2-3 min.)
Beat in the egg and vanilla until well combined.
Add in the flour, baking powder, and salt. Use a rubber spatula to fold the ingredients together just until no flour streaks remain.
Cover the dough and leave it in the fridge to chill for at least 30 minutes, or for up to 2 days.*
Preheat the oven to 350°F / 175°C. Line a cookie sheet with parchment paper or with a silicone baking mat.
Use a 3 tbsp. cookie scoop to dish out the cookies onto the prepared cookie sheet. Evenly space the cookies apart to allow room for spreading. Bake for 10-12 minutes, until the sides of the cookies start to turn golden brown.
Immediately after taking the cookies out of the oven, you can take a bowl that is slightly larger than the cookies and swirl it around over the cookies to make them a little more uniform and circular.
Use the bottom of a pinch bowl or shot glass (~2-3") to indent the center of each cookie while they are still warm.
Twix Topping
Place 1 tbsp. of salted caramel sauce in the center of each cookie, then freeze the cookies for 10 minutes to let the caramel harden a little bit.
In a small bowl, melt together the chocolate chips and vegetable oil. Put the chocolate in the microwave for 30 second intervals, stirring before putting it back in the microwave.
Top the caramel with the chocolate and some sea salt flakes if desired.
Allow time for the chocolate to harden (this can be sped up by placing them in the fridge), then serve and enjoy!
Notes
Affiliate links: Kitchen Scale (my favorite) | Kitchen scale (great starter option) | | Oven Thermometer | Cookie Scoops | Cookie Sheet Pan | Milk Pan |*You can make the dough and keep it stored in an airtight container in the fridge for up to 2 days before baking it. Just be sure to bring the dough out to sit at room temperature for a little bit, until it becomes soft enough to scoop again. For longer term storage, scoop out the dough balls and freeze them on a baking sheet or tray. Once they're completely frozen, you can transfer them to a ziploc or airtight container and keep them frozen for up to 1 month. I'd recommend letting the dough sit out for 20 min. before baking it, or thawing it in the fridge overnight before baking it. Please note that your cookies will likely bake up a little thicker.