Small aspic cutters are ideal for making the "windows" in the tops of the sandwich cookies.
In a large bowl, sift together the flour, salt, and baking powder; set aside.
Place the butter and the sugar in the bowl of an electric mixer fitted with the paddle attachment. Mix on medium-high speed until mixture is light and fluffy, about 2 minutes. Reduce the mixer speed to medium; add eggs, one at a time, mixing well after each addition. Mix in the vanilla.
Add the reserved flour mixture, and mix on low speed until combined. Turn out the dough, and flatten into a disk. Wrap dough in plastic. Chill 30 minutes.
Heat oven to 325 degrees. On a lightly floured surface, roll out dough to an 1/8-inch thickness. Cut cookies into desired shapes, using smaller cookie cutters or aspic cutters to form decorative tops. Transfer cookies to two ungreased baking sheets. Chill until firm, about 15 minutes.
Bake cookies until edges start to brown, 15 to 18 minutes, rotating halfway through baking. Transfer to a wire rack to cool.
Place the jelly in a fine sieve set over a small bowl. Using the back of a ladle, push the jelly through the sieve. Using a small offset spatula, spread a thin layer of jelly over half of the cookies; place the remaining cookies on top, forming the sandwiches.
Place the royal icing in a pastry bag fitted with small round tip, such as a Wilton #3. Decorate and pipe messages on the tops. Allow icing to dry. Store the cookies in an airtight container up to 1 week.