Combine brown sugar and butter. Spread into greased 9 inch round baking pan. Layer with rhubarb, sprinkle with sugar.
Cream butter and sugar. Beat in egg yolks and vanilla. Combine flour, baking powder, and salt. Add to creamed mixture alternately with milk. In a small bowl beat egg whites and cream of tarter on medium speed until stiff peaks. Fold into the creamed mixture (2 1/2 cups at a time) Gently spoon over rhubarb. Bake at 325 degrees for 40-45 minutes. Cool 10 minutes and invert.