In a large mixing bowl, cream together the butter, granulated sugar, and powdered sugar until smooth. Add the egg and vanilla extract and beat well.
Mix in the flour, almond flour, and salt until the dough forms and no dry streaks remain.
Wrap the dough in plastic wrap and refrigerate for at least one hour.
Preheat the oven to 350°F (180°C), or 160°C fan oven. Line 4 cookie sheets with parchment paper or a silicone mat and set aside.
On a lightly floured surface, roll the dough until it is about ¼” thick, then use your cookie cutters to cut out desired shapes, half solid and half with a cutout “window” in the middle.
Carefully transfer the cookies to the prepared baking sheets, and bake for 8-10 minutes or until lightly golden.
Allow to cool completely, then flip the solid cookies so that the cooked side is facing up. Spread with a thin layer of desired jam, then press the cutout cookie on top. Sprinkle the cookies with powdered sugar before serving.
Notes
Pay attention to your measurements. Dry or crumbly cookie dough is almost always an issue of flour being over-measured. Be sure to measure the flour by scooping and leveling, rather than dipping it into the flour canister.
Chill the dough. For cut-out shortbread cookies that won't spread while baking, the butter in the dough needs to be cold. Chill this dough for at least 1 hour, or longer.
Don't overbake. These cookies are meant to be light and delicate. You'll know that they're done when the edges just start to turn brown.
Storing. Keep these in an airtight container at room temperature for up to 4 days, or freeze for up to 3 months. For best results, wait to fill the cookies with jam until just before serving.