Bake the boxed/packaged cake mix according to the instructions using a fluted cake pan (or follow your own favourite homemade recipe for white bundt cake). Be sure to grease your pan thoroughly so it doesn't stick.
Invert onto plate or cakestand and allow to cool completely.
Add butter and brown sugar to a medium saucepan over low heat. Bring to a boil for 2 minutes, stirring occasionally.
Whisk in milk (or half and half) and bring to a boil stirring constantly. Remove from heat and allow to cool for about 15 minutes.
Stir in powdered sugar and mix thoroughly so there aren't any lumps. Transfer to a pourable container.
In a back and forth motion over the top of the cake, drizzle glaze all around (turning the plate as you go). It should drip down the sides slowly (if it's dripping too quickly, the glaze is likely too warm).
Quickly sprinkle top of cake with toasted pecans before the glaze sets. Allow to cool.