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- 7 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 center of each cookie with thumb or with or the rounded back of a teaspoon (edges may crack slightly). Refrigerate at least 1 hour (see notes)
Preheat oven to 350º F (180º C).
Fill each indentation to the brim with jam, about ¼ - 1/3 teaspoon each.
Before baking, make sure they are 2-inch apart.
Bake for 14-16 minutes, or until edges are lightly browned. Let stand 1 minute on cookie sheets; remove to cooling rack.
Combine all glaze ingredients in a bowl with whisk until smooth. Drizzle over cookies.