Beat the softened butter and granulated sugar on medium speed until light and fluffy, about 3 minutes. Scrape down the bowl once or twice. You're looking for a pale, airy mixture - this is what gives the cookies their tender crumb.

Beat in the egg yolks one at a time, then add the vanilla. Mix just until combined. The dough will look slightly curdled at first but will smooth out.

Add the flour and salt all at once. Mix on low speed until the dough just comes together and no dry flour remains. It will look sandy at first, then form a cohesive mass. Do not overmix.

Scoop level tablespoons of dough and roll into balls. Place on parchment-lined baking sheets about 2 inches apart. Chill the whole tray for 20 minutes. This firms the butter so the cookies hold their shape.

Use your thumb or the back of a 0.5 teaspoon measure to press a deep well into each ball, going almost to the bottom but not through. The walls should be thick enough to hold the jam.

Fill each well with 0.5 teaspoon of raspberry jam. Do not overfill or it will bubble over. Bake at 350°F for 12-14 minutes, until the cookies are set and just barely golden on the bottom edges. They will look underdone - that's correct.

Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely. The jam needs to set before moving or it will run. Dust with powdered sugar just before serving.