Preheat and prep: Heat your oven to 350°F (175°C). Line a 12-cup muffin pan with paper liners.
Mix dry ingredients: In a bowl, whisk flour, sugar, cocoa powder, baking soda, and salt until combined.
Mix wet ingredients: In a separate bowl, whisk egg, oil, buttermilk, vanilla, vinegar, and food coloring until smooth and evenly red.
Combine batter: Pour the wet ingredients into the dry.
Stir gently until just combined. Don’t overmix—stop when no dry streaks remain.
Bake: Divide the batter among cupcake liners, about 2/3 full. Bake 16–19 minutes, or until a toothpick comes out clean.
Cool completely on a rack.
Make frosting: Beat cream cheese and butter on medium until smooth and fluffy, about 2 minutes. Add vanilla and salt. Gradually mix in powdered sugar until thick, creamy, and pipeable.
Adjust with more sugar for thickness or a teaspoon of milk for softness.
Crumble or slice the cupcakes: Remove liners. Either crumble cupcakes into soft chunks or slice them horizontally into rounds that fit your jars. Crumbles give a rustic look; rounds give tidy layers.
Layer the jars: Add a layer of cake to the bottom of each jar.
Pipe or spoon a layer of frosting over it. Repeat layers until you reach near the top, finishing with frosting.
Garnish: Add sprinkles, shaved chocolate, or a few crumbs of red velvet on top for contrast.
Chill briefly: Cover and refrigerate for 30–60 minutes to set the layers. Serve chilled or at cool room temperature.