Blend sugar with flour and salt. Put in top of double boiler and add milk. Cook until slightly thickened, stirring canstantly. Beat egg yolks; add a small amount of cooked custard and mix well. Add to custard and cook about 2 minutes stirring until mixture is thick. Add vanilla. Line buttered casserole dish with wafers, then a layer of sliced bananas.Cover with a layer of custard, repeat until all custard is used. Bake in oven for about 20 minutes at 325 degrees.