Preheat oven to 350 degrees. Line and spray three 9-inch pans.

Whisk the 3 1/2 cups flour, salt, baking powder, and baking soda together. Set aside.

Beat the 1 2/3 cups sugar and 1 1/2 cups soft butter on high until smooth and creamy (2-3 minutes), scraping down the sides and bottom.

Beat in the eggs, egg whites, and 1 Tbsp vanilla until combined (batter will look curdled).

Slowly add the dry ingredients, mixing on low. Slowly add the buttermilk, mixing just until combined. Pour into the prepared pans.

Mix the brown sugar, 1/3 cup flour, and 1 Tbsp cinnamon together, then pour the melted butter over and mix. Drop spoonfuls onto the unbaked cakes and swirl with a knife.

Bake for 23-26 minutes until a toothpick comes out clean. Cool in pans for 15-20 minutes, remove to finish cooling, then wrap in plastic wrap and freeze 4 hours or overnight.

Mix the heavy cream, 1/2 cup powdered sugar, pudding mix, and 1 Tbsp vanilla in a cold bowl. Whisk on high until thick (30-45 seconds) and place in a large piping bag.

Beat the 3 sticks of very soft butter with a paddle attachment for 5-6 minutes until almost white.

Add 2 cups of powdered sugar to the butter and mix on low. Add 2 more cups and 2 tsp vanilla, mixing on low. Add the last 2 cups of sugar, the ground cinnamon, and 2 Tbsp liquid. Add remaining liquid 1 Tbsp at a time until smooth.

Take cakes out 20-30 minutes before frosting. Pipe a line of buttercream around the edge of the first layer as a dam, and fill with the whipped vanilla filling. Repeat with the second layer.

Frost the cake with a crumb coat, place in the fridge for 5-10 minutes, then finish frosting. Garnish with mini cookies.