Step 1Combine cornstarch, sugar and salt in saucepan. Stir in boiling water until blended. Bring to boil, stirring until mixture is clear, about 2 minutes. Place cornstarch mixture in top of double boiler set over, but not touching, simmering water. Cook 10 minutes, stirring occasionally. Add small amount hot mixture to beaten egg yolks, then return all yolk mixture to pan, stirring until blended. Cook 2 minutes. Remove from heat. Stir in lime juice, butter and zest.
Step 2Pour filling into Master pie shell. Spread meringue over filling, being careful to seal to edges of pie. Bake at 400 degrees until golden brown, 8 to 10 minutes. Cool to room temperature, about 1 hour.