I love that the cookies are incredibly tasty and provide such a sweet surprise inside. Win-win.
Technique tip: Take your time in layering and chilling the cookie batter before cutting and stamping out shapes. You'll get a more beautiful finished cookie. Keep the cookie cutter handy after the bake so you can clean up the hot cookies if they've spread.
Swap option: Use sprinkles or sparkling sugar! It's all fun and sweet.
Special equipment: Heart shaped cookie cutters (2 sizes) and piping bag
For the white dough:
In the bowl of a stand mixer, combine the butter and sugar. Beat on high speed until light and fluffy. Scrape down the bottom and sides of the bowl and add the eggs, one at a time, beating well after each addition.
In a mixing bowl combine the flour and salt; whisk to combine. With the mixer on low, add the vanilla to the butter mixture, then gradually add the flour mixture until just combined (don't over mix). Transfer dough to a piece of plastic wrap and press into a rough rectangle. Wrap and chill for 20 minutes.
For the red dough:
In the large bowl of a stand mixer combine the butter and sugar. Beat on high speed until light and fluffy. Scrape down the bottom and sides of the bowl and add the eggs, one at a time, beating well after each addition.
In a medium mixing bowl combine the flour and salt; whisk to combine. With the mixer on low, add the vanilla and food coloring to the butter mixture, then gradually add the flour mixture until just combined (don't over mix). Transfer dough to a piece of plastic wrap and press into a rough rectangle. Wrap and chill for 20 minutes.
For the cookies:
Preheat oven to 325°F. Line baking sheets with parchment paper; set aside. Divide each piece of dough in half. Roll each piece of dough into a 12- by 8-inch rectangle on a piece of plastic wrap or waxed paper. Invert a red rectangle on top of a plain rectangle and remove top sheet of plastic wrap. Invert remaining plain rectangle on top of the stack, removing plastic wrap. Add remaining red rectangle, and remove plastic wrap. Cut the dough crosswise into four 8- by 3-inch strips. Stack one strip on top of the other and press gently to adhere (you should have 16 layers total). Wrap in plastic wrap and chill at least 30 minutes or up to 1 1/2 hours.
Using a very sharp knife, trim the short ends of the stack. Cut a 1/4-inch slice from the short end of the stack. Press the slice with your hand until even and the layers hold together.
Using a 3-inch heart-shaped cookie cutter, cut a striped heart from the slice. Transfer to prepared baking sheet. Repeat with remaining stack (you should have about 28 striped hearts). Gather the scraps and pat together. Reroll to 1/4-inch thickness and cut 3-inch hearts.
Place all the hearts on a parchment paper-lined baking sheet and bake for 10 to 12 minutes or until edges of the cookies are very lightly browned. While still warm, cut again with clean 3-inch cutter. Using a small 1½-inch heart-shaped cookie cutter, cut a small heart from the middle of 14 marbled hearts. Remove and cool on wire racks.
For the royal icing:
In a small bowl stir together the powdered sugar and egg whites. Add water, one teaspoon at a time, until mixture resembles a thick, pipeable paste. Add a small drop of red gel food coloring, if using, and stir to combine. Transfer to a small pastry bag fitted with a small round tip. Place a damp paper towel into the bottom of a tall glass and place the bag with royal icing with the tip resting in the towel to keep icing from hardening.
For the assembly:
Place half of the striped cookies top-sides down; pipe a thin ribbon of royal icing around the inside edge of each. Place a marbled cookie with cutout on top of each piped cookie, aligning edges. Let dry about 2 minutes. Spoon valentine sprinkles into the cutout in each. Pipe another thin strip of royal icing along the inside edge of each and top with remaining striped cookies, top sides up. Allow to dry completely, about 10 minutes.