In a large bowl, whisk together the flour, corn starch, powdered sugar, baking powder and salt, until well combined.
Add the cold butter pieces, and use a pastry cutter to cut the butter into the flour mixture until very small pieces of butter remain, and the mixture is crumbly.
Add the vanilla, almond extract and egg yolks, and stir in to moisten the dry ingredients, then use your hands to work the moisture in until it comes together into a soft dough.
Divide the dough into 18 equal portions. TIP: This is easiest to do if you weigh all the dough, divide that number by 18, then weigh out each portion. So if you have 21 ounces of dough, you would weigh out around 1.2 ounces of dough per cookie.
Roll each portion of dough into a ball, then coat the dough balls in the 1/4 cup of granulated sugar. Press the bottom of a measuring spoon into each dough ball to indent the center.
Stir the raspberry jam thoroughly, so that it's very smooth, not chunky. Fill the centers of each of the cookies with about 1 teaspoon of the jam.
Chill the unbaked cookies in the refrigerator for 1 hour, to help prevent them spreading too much as they bake.
Preheat the oven to 350 F, and position a rack in the center of the oven. Line a large baking sheet with a piece of nonstick baking paper or parchment paper.
Place the chilled cookies 3 inches apart on the baking sheet. Bake for about 11 minutes, just until the edges are set, and the color is a pale golden brown. Cool the cookies on the baking sheet for 2 minutes, then transfer to a wire rack to cool completely.