In non stick pan, mix sugar, flour and salt. Gradually add milk and egg yolks, stirring until well blended. Place pan on low heat. Stir until mixture starts to thicken. Do not over cook -- mixture will thicken when it cools.
Remove from heat and add margarine and vanilla. Stir until creamy. Cool, and then pour in baked pie crust
Beat egg whites until stiff. Add sugar and vanilla. Continue beating until well blended. Spread on cooled filling. Sprinkle with coconut. Brown in 450 degrees F oven. Cool well before serving.