Heat your oven to 350°F and line two cookie sheets with parchment paper. This gives you everything ready to go when your dough is prepared. I always preheat early because it's easy to forget once you're in the middle of mixing, and you want the oven at temperature when the cookies are ready to bake.

In a large bowl, beat the softened butter and granulated sugar together for 3-4 minutes until the mixture is light, fluffy, and pale in color. This creaming process incorporates air into the dough, which helps create the tender, crumbly texture characteristic of shortbread. Stop occasionally to scrape down the sides of the bowl to ensure everything is evenly mixed.

Add the room-temperature egg and vanilla extract to the butter mixture and mix well until fully combined, about 1-2 minutes. The room temperature egg incorporates more smoothly and doesn't shock the butter mixture. Scrape the bowl again to make sure no streaks of egg white remain.

In a separate bowl, sift together the all-purpose flour, cornstarch, and baking powder—sifting aerates them and prevents lumps. Slowly add this dry mixture to the creamed butter mixture, stirring gently until just combined and the dough is smooth. I prefer to add the dry ingredients in two additions rather than all at once, which helps prevent overmixing and keeps the shortbread tender. Mix just until no flour streaks remain, being careful not to overwork the dough.

Divide the dough in half and roll each half to 1/4 to 1/2 inch thickness on a lightly floured surface. Using a 2-3 inch cookie cutter, cut out rounds and place them on your prepared baking sheets, spacing them about 1 inch apart. Then, using a small 1/2-inch cutter, cut centers out of exactly half of the cookies—these will become your top cookies with the "window" to show the jam. Leave the other half of cookies whole as your base layers.

Bake for 12-18 minutes until the edges are light golden brown. The centers should still look pale and slightly underbaked—this ensures they stay tender as they cool and set. Keep an eye on them starting around the 12-minute mark, as oven temperatures vary. Remove from the oven and let the cookies cool completely on the baking sheet before handling them.

Once the cookies are completely cool, dust the cookies with the center cutout (the top cookies) generously with powdered sugar. Spread 1/2 to 1 teaspoon of strawberry preserves onto each solid cookie (the base layer), then carefully place a powdered sugar-dusted cookie on top, window-side up, so the jam shows through. The powdered sugar creates a beautiful presentation and adds a subtle sweetness that complements the jam and rich shortbread.