Combine 3/4 cup sugar, cornstarch, salt and milk in a heavy saucepan. Cook over medium-high heat, stirring constantly, until thick and bubbly. Continue to boil for one minute. Remove from heat.
In a medium bowl, beat the 3 egg yolks. Gradually stir in 1/4 of the hot mixture into yolks. Pour yolks back into remaining hot mixture stirring constantly. Cook, stirring constantly, 30 seconds. Remove from heat and add butter, vanilla and coconut. Pour into baked pie shell. Cool in refrigerator.
Preheat the oven to 375 degrees F (190 degrees C)
In a medium glass or metal bowl, beat egg whites until frothy. Slowly add 6 tablespoons sugar while continuing to beat until whites form stiff peaks. Spread meringue over pie, sealing to the edges and sprinkle with coconut.
Bake in the preheated oven until meringue is toasted, 10 to 15 minutes.