Preheat the oven to 375 degrees and butter a 9-by-13-inch baking pan. In a large bowl using a wooden spoon, cream the 1 stick butter with the brown sugar, egg and 1 teaspoon vanilla. In a medium bowl, mix together the flour, oats, raisins, walnuts, cinnamon, baking soda and salt and stir into the butter mixture until combined. Spread the dough in the prepared pan and press into an even layer with floured fingers. Bake until firm in the center, 15 to 20 minutes. Let cool completely in the pan.
Using an electric mixer or a wooden spoon, blend the cream cheese with the remaining 2 tablespoons butter, the confectioners sugar and the remaining 1 teaspoon vanilla until smooth. Spread on top of the cooled cake and cut into 24 bars.