Line two baking sheets with parchment.
In a large bowl, whisk together flour, cornstarch, and salt.
In another bowl, beat butter and sugar, scraping bowl often, until creamy, pale and fluffy, about 5 minutes. Add vanilla; mix to incorporate. Then add dry ingredients in two batches and continue beating at low speed, scraping bowl often, until well mixed.
Shape dough into 1-inch balls. You can also use a small ice cream and scoop 1” balls. Place balls onto prepared baking sheets. Make an indentation in the center of each cookie with your thumb or with or the rounded back of a teaspoon (edges may crack slightly). Refrigerate for at least 2 hours up to 4 days.
Preheat oven to 350º F (180º C).
Fill each indentation to the brim with jam, about 1/4 - 1/3 teaspoon each. Place them in the prepared baking sheet 2-inch apart. IMPORTANT NOTE: if you decide to fill the cookies with any type of chocolate, caramel, lemon curd, or dulce de leche the cookies must be filled after baking once they are completely cool.
Bake for 14-16 minutes, or until edges are lightly browned. Let stand 5 minutes on cookie sheets; remove to cooling rack.
Combine all glaze ingredients in a bowl with whisk until smooth. Drizzle over cookies, if desired.