Place a rack in the center of oven; preheat to 350ºF
Spray Baker's Joy on all sides of quarter sheet pan
Place cake mix, buttermilk, applesauce, oil, eggs, and cinnamon in large mixing bowl.
Blend with electric mixer on low until incorporated (I used my KitchenAid paddle)
Stop machine and scrape down the sides of bowl with a rubber spatula.
Increase mixer to medium speed and mix for two minutes more.
Scrape down sides of bowl again, if needed.
The batter should be thick and well combined.
Pour batter in the quarter sheet pan, spread evenly.
Lightly tap the pan on the counter, to remove air bubbles in the batter.
Cook for 30-35 minutes or until top of cake springs back, when touched.
Cool in the pan, on a cooling rack for 10-15 minutes, then flip the cake out onto some parchment paper and allow it to completely cool before icing.
For the Icing:
Place butter in a large mixing bowl; blend with electric mixer on low speed till fluffy.
Stop the machine; add sugar; 3 Tablespoons of the milk; vanilla; cinnamon
Blend on low speed till incorporated, 1 minute.
Increase speed to medium and beat until fluffy. If the icing seems stiff, add remaining Tablespoon of milk
Spread on cake.
Top with chopped pecans.