Linzer cookies are a twist off the linzer torte, originating from Linz, Austria. The linzer torte is one of the oldest tortes in the world, found in an Austrian abbey in the early 1700s. Luckily for us, the linzer torte was brought to the US around 1850s. The torte's crust is a buttery pastry with ground nuts (traditionally hazelnuts or almonds), egg yolks, spices and lemon zest; the filling was made from black currant preserves with a lattice crust.  
Linzer cookies use the same ingredients as a linzer torte, excepted it's an almond cookie sandwich with jam filling. In America, raspberry is the most common filling. The top cookie has a small cutout in its center called "Linzer eyes." Traditionally, the cutout is a circle but any shape can be used - diamonds and hearts are very popular. (Joy of Baking, Foodreference)
These are traditionally Christmas cookies in Austria/Europe, I also think they're perfect for Valentines, especially with the heart shape <3
Because I didn't learn from my last pasty attempt, this is also another bake-at-midnight-because-I-didn't-plan-ahead-of-time dessert. The recipe is adapted from King Arthur Flour. The first chilling made the dough very difficult to work with since it was so hard; I would probably try skip that step. The dough can be tricky to roll, so add flour as needed. However, do not skip chilling the dough after your make the shapes, just prior to baking. This helps the cookie cutouts retain their shape - which I learned the hard way - as I attempted to pair fat heart cookies with normal heart cookies.
Since this was a gift, I chose to double batch the recipe below. While this cookie takes some patience, you'll be well rewarded with happy people gobbling up your cookies. Happy Valentine's :)

Linzer Cookies

INGREDIENTS
3/4 cup unsalted butter
1/2 cup sugar
grated rind (zest) of 1 lemon, or 1 teaspoon ground cinnamon
1 large egg yolk
1 teaspoon vanilla extract
1 cup King Arthur Unbleached All-Purpose Flour
3/4 cup almond flour
1/4 teaspoon fine sea salt
raspberry jam for filling

DIRECTIONS
Beat the butter, sugar, and zest until light and fluffy, about 3 minutes. Beat in the yolk and vanilla.

In another bowl, whisk together the flour, almond flour, and salt. Add the flour mixture to the egg mixture and mix until just combined. Don't over-beat. Divide the dough in half, and form two discs. Wrap each disc in plastic wrap, and refrigerate until firm.

To assemble, roll one disc of dough out about 1/4"-thick on a well floured surface. Using the cookie cutter shape of choice, cut out cookies and cookie cutouts until dough is used up. Place cookies on a parchment-lined baking sheet. Chill the cut cookies in the refrigerator for 30 minutes.

Preheat the oven to 350°F.

While the first half of cookies is chilling, cut cookies and cookie cutouts from the remaining dough. Place cookies in the refrigerator for 30 minutes to chill.

Bake the first batch of cookies for 10 minutes, or until the edges are just beginning to turn brown (do not let the cookie turn too dark). Bake the second batch as well. Let them cool for 5 minutes on the pan before transferring to a rack.

To fill the cookies, place the cookies with the cutouts on a cookie sheet and sift confectioners' sugar over the top. Spoon jam into the center of the remaining cookies, spreading it slightly. Don't fill all thw way to to the edges or they'll ooze out then sandwiched. Top with the sugar-dusted cookies.