Beat on low speed until combined:
1 box white cake mix
1.5 oz instant vanilla pudding mix
3 eggs
1/3 c oil
1 1/4 c water
Beat on med-high for 3 minutes.
Grease the bottom of a 13x9 pan.
Pour 1/2 to 2/3 of the batter into the pan.
Combine:
scant 1/2 c brown sugar, don't pack
2 tsp cinnamon
Sprinkle over batter.
Spoon the rest of batter on top.
Cinnamon layer will not be completely covered.
Bake @ 350 for 30 minutes or until toothpick inserted in cake comes out clean.
Frosting:
1 c confectioner sugar
1 1/2 Tbsp butter
1/2 tsp vanilla
Add milk until spreading consistency.
