Whisk liquid sweetener, milk, and vanilla together in small bowl and set aside.
In bowl of electric mixer, cream butter, sugar, and salt until light and fluffy.
Beat in egg, scrape down sides of bowl, and mix for 10 seconds more
Add 1/3 of flour and mix, then 1/2 of vanilla mixture and mix.
Repeat again, twice, until all of the flour has been mixed until just combined.
Dollop over cookie base and spread carefully into even layer.