Combine 3/4 cup sugar and 2 Tablespoons flour in a bowl and mix well. Stir in the sour cream, egg, vanilla and cinnamon. Add the apples and mix gently. Spoon into the pie shell.
Bake at 375 degrees for 45 minutes.
Combine 1/2 cup flour, 1/2 cup sugar and cinnamon in a bowl and mix well. Cut in the butter until crumbly. Sprinkle over the baked pie.
Bake at 400 degrees for 10 minutes longer.
Serve warm or at room temperature with vanilla ice cream.