In a large bowl, whisk together flour, cornstarch, and salt.
Separately, with a hand mixer or stand mixer, beat together softened butter and Truvia® Sweet Complete™ until creamy.
Add egg yolk and vanilla, mixing to integrate.
Gradually add flour mixture in 2-3 batches, mixing as you go until combined. (It will seem dry and crumbly at first, but it will Scrape down the sides of the bowl and continue to mix, adding milk as needed to adjust texture. Use hands to press dough together and get it fully combined.)
Once dough holds together when pressed, it’s ready to roll into cookies.
To bake cookies:
Preheat oven to 375°F (190°C).
Scoop dough into 1-Tbsp size balls (I use a cookie scoop) and roll between palms until smooth, making sure there are no cracks.
Add dough balls to a prepared baking sheet, leaving space in between them, and press slightly to flatten each into a disc shape. Use a finger or the rounded end of a spatula to press an indent into each cookie, creating a small well in the center and smoothing out the edges as needed.
For Christmas cookies with jam filling: Add jam into the thumbprint indent of each cookie before baking, filling them to the brim.
For Thanksgiving cookies with caramel filling: Bake thumbprint cookies first, without any filling. Prepare caramel walnut filling (see below) while cookies bake, then add filling once cookies have cooled slightly.
Bake for 12-14 minutes, until golden brown.
Allow to cool before enjoying.
Cover cookies or store in an airtight container in the fridge for up to 10 days.
To make caramel walnut filling:
Add butter to a small saucepan over medium heat, until melted. Add Truvia® Sweet Complete™, stirring constantly until it integrates and comes to a boil.
Cook 1 minute over a boil, stirring regularly.
Remove from heat and stir in heavy whipping cream, followed by chopped walnuts and chocolate chips. Stir well until chocolate chips melt and integrate.
Allow to cool for about 10-15 minutes, until it’s a caramel consistency, then immediately spoon mixture into the center of baked cookies. (You don’t want to wait too long or the caramel may harden a bit!)