Prep. Preheat oven to 350°F (177°C) and line a large baking sheet with parchment paper, and set aside.
Mix dry ingredients. Add flour, cornstarch and salt into a medium bowl and whisk to combine.
Cream butter and sugar. Place butter in a large bowl and beat with an electric beater, on high until creamy and fluffy (about 2 minutes). Scrape the sides of the bowl, add confectioners sugar and vanilla extract and beat for an additional one minute.
Finish dough. Scrape down the sides of the bowl, then add the flour mixture and beat on low speed until the dough looks moist and crumbly. Using your hands, gently knead the dough a few times until it comes together into a soft, smooth ball.
Make balls. Scoop the dough into 1-tablespoon portions, roll each one into a smooth ball using the palms of your hands, then gently flatten into a ½-inch thick disc. 👉 Make sure there are no cracks—if any appear, knead the dough ball briefly with your hands; the warmth will help soften and smooth it. Roll in sugar. Place the granulated sugar in a small bowl, roll each dough disc in the sugar to coat, and arrange them on a parchment-lined baking sheet, spacing them evenly apart.
Make indent. Use the rounded back of a one-teaspoon measure (or your thumb) to gently press an indent in the center of each cookie dough ball. Repeat with the remaining dough, and if any cracks form around the edges, simply pinch them back together. Space them about 1-inch apart.
Fill with jam. Place the jam in a small, microwave-safe bowl and heat for 5–10 seconds, just until it loosens—be careful not to make it hot. Spoon about ½ to ¾ teaspoon of jam into each cookie indent, filling it to the top.
Bake. Bake for 15 to 18 minutes or the edges turn golden brown. Place the baking sheet on a wire rack to cool completely.