Make the Cake

1. Preheat the oven to 350°F. Grease and flour two 8-inch pans.
2. Combine the vanilla cake mix with the vanilla pudding. Add the eggs and egg white, followed by buttermilk, oil, sour cream, and vanilla extract.
3. Beat on medium speed until all the ingredients are well combined. Use a spatula to scrape down the side of the bowl and turn the batter over from the bottom until well mixed. Set the batter aside and prepare the streusel.

Make the Streusel

1. Combine all dry ingredients in a medium-sized bowl. In a separate bowl, melt the butter. Pour the melted butter over dry ingredients. Use a pastry cutter or fork to blend butter into dry ingredients to create a crumbly mixture.
2. Layer the cake batter and the streusel, swirling together with a knife. Any remaining streusel can be sprinkled over the top.
3. Bake at 350°F for 35 to 38 minutes until the cakes are done. Check for doneness by inserting a toothpick in the middle of the cake. If the toothpick comes out clean, your cake layers are done. Allow complete cooling and then “tort” the top of the cake, cutting off the dome so it is completely flat. Then cut each layer in half.

Make the Frosting

1. Allow butter to come to room temperature so it is soft but not melted. Beat butter in mixer on medium speed until smooth and creamy for 3 minutes. Add the brown sugar and continue mixing for another 2 minutes.
2. Slowly add powdered sugar 1 cup at a time while continuing to beat on medium speed. Add cinnamon and mix into the frosting.
3. As the frosting starts to thicken, alternate adding the vanilla extract and heavy whipping cream. Increase the mixer speed to high and whip the frosting for another 3 minutes, until it’s smooth and creamy.

Assemble

1. Place a dollop of frosting on the bottom of the cake plate and place the bottom layer of cake on top. The frosting will help prevent the cake from sliding.
2. In order to create even layers, I use a piping bag fitted with an open round tip, or simply cut off the end of the piping bag. I also highly suggest using a lazy Suzan to rotate the cake as you work.
3. Fill the first layer of frosting by using your piping bag and pipe from the outside edge into the center. Gently spread the frosting with an offset spatula to fill in the gaps.
4. Place the next layer of cake on top and repeat the above instructions to fill the layer of frosting. Repeat a third time. For the top layer of cake, place it inverted, so the bottom of the cake is actually the top, creating a nice and smooth top edge.
5. Proceed to add your crumb coat by adding a cup of frosting to the top of the cake and gently spread out to the edges and down the sides. Use the leftover frosting in your piping bags to fill in the cracks between the layers.
6. Using an offset spatula or icing smoother, wipe away excess frosting, leaving you just enough to coat the outside of the cake. Do not mix your crumb coating frosting back into your icing if it has cake debris in it. Refrigerate the cake for 15-20 minutes to allow the crumb coat to harden.
7. Remove the cake from the refrigerator and proceed to the remaining frosting to the top and edges of the cake. To get a nice smooth edge, use your offset spatula and icing smoother to wipe away excess frosting.
8. Once you are satisfied with the frosting of the cake, crush 5 Snickerdoodle cookies into crumbs and press the crumbs around the bottom of the cake, and sprinkle the remaining cookies on top.