Whisk together flour, cocoa, baking soda, and salt in a small bowl.

Beat butter and sugar 2 minutes until pale. Add egg, food coloring, and vanilla; beat until combined. On low, add dry mixture alternately with buttermilk in three additions, starting and ending with dry. Stir in vinegar by hand until just smooth.

Preheat oven to 350°F (175°C). Grease two 4-inch round pans and line with parchment. Divide batter evenly between pans. Bake 18 to 22 minutes, until a toothpick comes out clean. Cool 10 minutes in pans, then turn onto a rack to cool completely.

Beat cream cheese and butter until smooth. Add powdered sugar and vanilla, beat on low to combine, then whip on medium until fluffy, 1 to 2 minutes.

Place one cake layer on a serving plate, spread about 3 tablespoons frosting on top. Add second layer. Apply a thin crumb coat all over, chill 15 minutes. Frost with remaining frosting, smoothing the sides.

Pipe rosettes or a border if you like, scatter on sprinkles or crumbs, and serve.