Preheat oven to 425 degrees F. Butter two 8-inch diameter cake pans. In small bowl, mix together brown sugar, cinnamon, walnuts, and currants or raisins, and set aside (filling). For biscuits, sift the flour, granulated sugar, baking powder, baking soda, and salt into large bowl. Add the 1/2 cup butter and blend it (either with a pastry blender or your fingertips) until the mixture resembles fine, irregular crumbs. Add the buttermilk and stir until the dough forms a cohesive mass. Turn the dough onto a floured work surface and knead gently about 10 times. Push and pat the dough into a rectangle about 12 inches long and 4 inches wide. Brush with 3 tablespoons of butter. Spread the filling over the dough and press it in gently. Starting at one of the long sides of the rectangle, roll the dough into a tight cylinder. Using a sharp knife, cut the roll into slices about 1 inch thick. Place the slices, cut side down and about 1/2 inch apart, in the prepared pans. Bake until the biscuits are puffy and lightly browned (about 15 minutes). Serve warm... and enjoy!