Add 1 cup of the confectioners' sugar and the vanilla. Beat at a low speed to incorporate. If room is very warm, filling may need to be chilled for about 20 minutes. It should be fluffy and light-textured, but it must not be runny. Just before you are ready to fill the cookies, lay out several long sheets of wax paper on a flat work surface.
Place cookies, flat side up, on wax paper.
Try to pair up cookies that have approximately the same size and shape. If desired, allow sandwiches, still wrapped, to come to room temperature before eating. Variation Ice Cream Sandwiches About 2 dozen sandwiches Make cookies as above, but omit filling. I suggest using an ice cream that isn't rock-hard at freezer temperature, just for ease in eating. For me, this means a variety of Starbucks ice cream, but use what you like.