In a large bowl, cream together the butter and brown sugar. Beat in the egg and water. Sift together the flour, baking powder, and salt; stir into the butter mixture until well blended.
On a lightly floured surface, roll out the dough to 1/4 inch thickness.
Cut with a 2 inch round cookie cutter. Put half of the cookies onto a cookie sheet and spread 1/2 of a teaspoon of plum jam in the center of each one. With a thimble, or small cookie cutter , cut the center out of the remaining cookies.
Place these on top of the jam topped cookies to make sandwiches. Press together.
Bake cookies for 10 minutes then remove to a rack to cool.
