Everyone's Favorite Birthday Cake

Preheat oven to 350 degrees. Butter two 8-inch round cake pans; line with parchment-paper rounds. Butter parchment paper and lightly dust pans with all-purpose flour, shaking out excess.

In a large bowl, sift together cake flour, baking powder, salt, and cinnamon; set aside.

In the bowl of an electric mixer fitted with the paddle attachment, beat butter and both sugars on medium speed until light and fluffy, about 2 minutes. Add eggs, one at a time, and beat until just incorporated, scraping down sides of bowl as necessary. Reduce speed to low; add flour mixture in three additions, alternating with sour cream and beginning and ending with flour mixture, scraping down sides of bowl as necessary.

Pour batter into prepared pans and bake until a toothpick inserted into the center comes out clean, 30 to 35 minutes. Set pan on a wire rack to cool for at least 20 minutes before loosening sides of cake from pan and inverting onto a wire rack. Remove parchment paper and reinvert; let cool completely.

Beat butter in the bowl of an electric mixer fitted with the paddle attachment until smooth. Add cream cheese and beat until well combined. Add 2 cups confectioners' sugar and salt; beat until smooth. Add chocolate and mix until well combined. If frosting seems too loose, add additional confectioners' sugar, 1 tablespoon at a time, until it becomes firm. Do not over-beat. Frosting can be made up to 1 day in advance if stored in an airtight container in the refrigerator; let soften at room temperature before using.

Using a serrated knife, trim tops of cakes to make even. Place 1 layer on a cake plate; frost top with 1/2 cup frosting and top with remaining layer. Frost top and sides with remaining frosting to cover. Cake can be kept in an airtight container, refrigerated, for up to 3 days. Let come to almost room temperature before serving.