Eggless Thumbprint Cookies

In a large mixing bowl, combine flour, cornstarch, and salt. Whisk together and set aside.
2 ½ cups all-purpose flour, 2 ½ tablespoons cornstarch, 3/4 teaspoon salt
In a stand mixer with a paddle attachment, add softened butter, white sugar, and brown sugar. Cream together on low speed, about 1 minute.
1 cup unsalted butter, 1/3 cup white granulated sugar, 1/3 cup brown sugar
Pour in room temperature milk and vanilla. Mix until combined.
2 ½ tablespoons whole milk, 3/4 teaspoon vanilla extract
Pour in dry ingredients and mix until just combined, avoiding overmixing.
Line a cookie tray with parchment paper. Scoop out 1 tablespoon of dough and roll into a smooth ball. Roll the dough ball in white granulated sugar and place on the baking tray.
Repeat until the entire tray is filled with rolled dough balls, placed 1 inch apart.
Press down slowly on the top of each ball with the back of a 1/2 teaspoon measuring spoon or your thumb, pressing about halfway down to make an indent for the jam.
Place the tray in the freezer for 30 minutes to prevent the dough from spreading.
During the last 10 minutes in the freezer, preheat the oven to 350°F.
Remove the tray from the freezer and fill each indent with jam. You don't want to fill it higher than the edge of the cookie.
Bake 13-17 minutes until the bottoms of the cookies are golden brown. Cool on the tray 5 minutes, then move to a cooling rack.