1. Preheat the oven to 350 degrees F. Lightly grease a 13 by 9-inch glass baking dish and set aside.
2. In a large bowl, cream together butter and brown sugar until light and fluffy. Add the eggs one at a time, beating after the addition of each. In a separate bowl, sift together the flour, baking soda, cinnamon, and salt. Add to the wet ingredients, alternating with the sour cream and vanilla. Fold in the apples. Pour into the prepared baking dish, spreading out to the edges.
3. To make the topping, in a bowl, combine the sugar, flour, cinnamon, and butter, and mix until it resembles coarse crumbs. Sprinkle the topping over the cake and bake until golden brown and set, 35 to 40 minutes. Remove from the oven and let cool on a wire rack for at least 10 minutes.
4. To make the glaze, in a bowl, combine the sugar, vanilla, and water and mix until smooth. Drizzle the cake with the glaze and let harden slightly. Cut into squares and serve.