Preheat oven to 350 degrees.
In a large bowl, mix water, pumpkin, eggs, vanilla, and pumpkin pie spice. Mix in cake mix (just the dry mix, not the ingredients listed on the box) and baking soda and mix until combined.
Pour batter into a greased 9x13 baking pan.
In a small bowl, mix together brown sugar, flour and melted butter. Sprinkle over the top of the cake (I found it was easiest to use my fingers to sprinkle this on top).
Bake for 25-30 minutes or until an inserted toothpick comes out clean.
In a small saucepan over medium heat, add glaze ingredients (1/2 cup brown sugar, heavy cream, white sugar, and 1 teaspoon vanilla extract). Bring to a simmer. Remove saucepan from heat and whisk until brown sugar and white sugar are dissolved and glaze is smooth.
Pour glaze over cake. Serve cake warm or it's delicious at room temperature as well.