Cream butter in a large mixing bowl; gradually add 1 cup brown sugar, beating at medium speed of an electric mixer until the mixture is light and fluffy.
Combine oats, flour, soda and salt; stir well. Add to the creamed mixture alternately with the buttermilk, beginning and ending with the flour mixture.
Drop dough by tablespoonfuls onto ungreased cookie sheets. Bake at 350 degrees for 12 to 15 minutes. Let cool slightly on the cookie sheets; transfer to wire racks and let cool completely.
Combine the chopped dates (or raisins), 3/4 cup sugar, boiling water, and vanilla in a small saucepan. Cook over low heat, stirring constantly, until the mixture is thickened. Remove from the heat, and let cool. Spread half of the cookies with the date mixture. Top with the remaining cookies.