Special equipment: a 4-inch rectangular cookie cutter; a 2-inch rectangular cookie cutter; 4 piping bags or decorator squeeze bottles; cellophane bags; baker's twine
For the cookie dough: In a stand mixer fitted with the paddle attachment, mix the butter and granulated sugar together until just incorporated. Add the egg and vanilla extract. Mix again on low speed, stopping to scrape down the sides of the bowl intermittently.
Add the flour and salt. Mix on low speed until a dough is formed and there are no longer streaks of butter in the mixing bowl. The dough will be thick and should not be sticky.
Cover two baking sheets with parchment paper.
Divide the dough in half, place one half on a lightly floured work surface and roll to a thickness of slightly greater than 1/4 inch. Cut out a total of 15 cookies using a 4-inch rectangular cutter. Put 7 on one baking sheet and 8 on the other. Cut out the centers from 5 of the cookies using a 2-inch rectangular cookie cutter. Chill the cut-outs for 20 minutes. Wrap the remaining dough in plastic wrap and refrigerate until ready to use.
Preheat the oven to 350 degrees F.
Bake the cookies for 12 to 15 minutes or until golden brown around the edges. Remove the cookies to a wire rack to cool completely. Let the baking sheets cool, then re-line them with parchment. Repeat the rolling, cutting, chilling and baking with the remaining dough half, for a total of 30 cookies, 10 with cut-out centers.
For the royal icing: In the bowl of an electric mixer fitted with the whisk attachment, mix the confectioners' sugar and meringue powder on low speed until combined. Add the water and beat on medium-high speed until very stiff peaks form, 3 to 5 minutes. Add the lemon extract and beat on low speed until combined. Place 1/4 cup of icing into each of three bowls. Cover the remaining icing in the mixer bowl with a damp paper towel to prevent drying. Tint one bowl of icing with red, one with pink and one with black (leave the icing in the mixer bowl white). Add drops of water to all 4 bowls to thin the icing to flood consistency. (Check the consistency by dragging a spoon through the center of each icing; when the icing is perfect flood consistency, the indentation should completely disappear in 10 seconds. Thin with more water, or thicken with additional confections' sugar as needed.) Transfer each color of icing to a separate piping bag or decorator squeeze bottle.
Cut a sheet of parchment paper into 10 small strips, each about 4 inches by 2 inches. Write sentiments of love on one side of each strip, such as "Love you more than cookies!" and "You're Sweet!"
Place 10 whole rectangular cookies on a work surface covered with parchment paper. Pipe white royal icing around the outer top edge of each cookie. Place a cookie with a hollow center on top of each iced cookie. Fill each with 1 tablespoon confetti heart sprinkles. Pipe white royal icing around the outer top edges of the hollow cookies. Fold the love notes so the ink side is on the inside of the fold and will not touch the cookies. Insert a folded love note into each hollow cookie and top with a whole cookie.
Pipe a line of white royal icing around the outer top edges of the cookies and flood the inside so that icing completely covers the top of the cookies. If there are gaps in the icing, use a toothpick to push the icing into the gaps so they are filled. Let the iced cookies stand until completely smooth and dry, about 3 hours.
When the icing is dry, use the white royal icing to pipe a triangular envelope flap on half (five) of the cookies. Use a toothpick to scrape away the point of each triangle to make room for the seal decoration. Use the red royal icing to pipe a small seal in the middle of each envelope. Pipe white royal icing lines down from both sides of the envelope flaps to the bottom corners of the cookies to create envelope folds. Use the pink royal icing to pipe several small hearts on the envelopes. Use the food color marker to draw a perforated line across the center of the cookies with "open here" instructions.
On the remaining cookies, pipe three equal-sized horizontal lines using black royal icing on the front centers of the cookies for address lines. Use the red royal icing to make a heart in the upper right-hand corner of the cookies for a postage stamp. Use the black food color marker to draw wavy lines to the left of the stamps. Further embellish the cookies with X's and O's in the top left corner, and add the name of the recipients to the top address line. Draw half-circle postage marks on the sides of the cookies using the food color marker or leftover black royal icing.
Let the cookies stand until set, about 3 hours. Package the cookies in cellophane bags tied with baker's twine. If necessary, include opening instructions so the recipient will be aware of the messages inside the cookies.