Preheat oven to 350°.
Spray a 9 x 13 inch cake pan with non stick cooking spray
In a mixer bowl, add cake mix, butter, eggs and milk.
Use an electric mixer at low speed until ingredients come together, raise to medium speed and mix about 2 minutes. Scrape down the sides and continue to mix until thoroughly combined.
Reserve one cup of cake batter and place in a separate bowl.
Pour remaining cake batter into the cake pan, spreading evenly.
Add chopped pecans, cinnamon and brown sugar to reserved cake batter, stirring until combined.
Dollop cinnamon mixture over cake batter.
Swirl with a knife creating a marbled swirl.
Bake for 28 -30 minutes or until a toothpick inserted in the center comes out mostly clean.
Cool on a wire rack for about 10 minutes.
In a bowl, add powdered sugar, butter and milk mixing until smooth.
Pour over warm cake, spreading evenly.
Slice and serve warm or at room temperature