Rhubarb Custard Pie

4 cups finely diced rhubarb
1 1/2 cup sugar
1/4 tsp. salt
3 Tbsp. flour
2 egg yolk
5 Tbsp. milk
1 Tbsp. butter

Cover rhubarb with boiling water. Drain well and arrange in pie shell. Beat egg yolk slightly, add milk, flour which has been sifted with sugar and salt, and mix well. Pour over rhubarb. Bake 425 for 10 minutes, then reduce to 375 and bake for 30 min. Dot top with butter.