Preheat oven to 350 degrees F. Take out butter and all wet ingredients at least 30 minutes or more before you start, they need to be at room temperature.Butter and line the bottoms of 3 8-inch cake pans with parchment paper. Butter parchment and the sides of the pans.Whisk together flour, baking powder, and salt and set aside.In a large mixing bowl or the bowl of a stand mixer, cream butter and sugar until very light and fluffy, 3 to 5 minutes, scraping down the sides of the bowl at least once.Add sour cream and vanilla and mix well.Add eggs, followed by egg whites, mixing well after each addition.Add half of dry ingredients and mix on low speed until just incorporated, then add half of milk and mix. Scrape down the sides of the bowl. Add remaining dry ingredients, followed by remaining milk, mixing until just incorporated.In a small bowl, whisk together cinnamon and sugar. Divide half of batter evenly among prepared pans. Sprinkle an even layer of cinnamon sugar over batter. Top with dollops of remaining batter, then gently spread into an even layer with an offset spatula, running the spatula through the batter once or twice to swirl the sugar (don’t overdo this or you’ll mix in the sugar instead of swirling it).Bake for 25 to 30 minutes or until edges are barely starting to turn brown and a toothpick inserted near the center comes out clean. Transfer to a wire rack to cool for 15 minutes, then turn cakes out of pans and let them cool completely, then wrap tightly in plastic wrap and chill. At this point you can also freeze the layers, wrapped tightly in plastic and stored in a zip top bag, until you are ready to assemble (freezing also makes the cakes easier to level/frost later, I do it even if I’ll be assembling the cake the next day).To prepare frosting, in a large mixing bowl, beat butter on medium-high speed until smooth and fluffy, 2 to 3 minutes. Add powdered sugar, 1/2 cup at a time, mixing well after each addition. As frosting thickens, add a few tablespoons of cream as needed. Depending on the temperature and consistency of your frosting, continue alternating additions of cream and powdered sugar, until frosting is light and fluffy. Mix in vanilla extract and food coloring (if desired), then increase speed and beat until almost whipped-cream like in consistency. You can make the frosting a day ahead of time; store in an airtight container in the refrigerator. Let the frosting fully come up to room temperature then whip it again in the mixer (with a smudge more cream if needed) before frosting your cake.To assemble, level each cake layer by cutting of the domed top with a long serrated knife. Place one layer, flat side down, on a cake stand or serving platter. Spread on about 1/2 cup of frosting onto cake using an offset spatula. Position second layer on top and press to adhere. Repeat with another 1/2 cup of frosting, and then position final cake layer, flat side up.Cover the entire cake with a thin layer of frosting. This “crumb coat” will keep the stray crumbs in place and make frosting the cake easier. Refrigerate for about 15 minutes to allow this crumb coat to set, then cover with remaining frosting, reserving some frosting for piping decorative details and borders if desired.Frosted cake will keep overnight, lightly covered or in an airtight container in the refrigerator or a cool, dry place. Sliced cake should be refrigerated in an airtight container otherwise it’ll dry out very quickly.