Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone baking mat. In a large mixing bowl, combine the chocolate cake mix, eggs, vegetable oil, and vanilla extract. Stir until a soft dough forms. Fold in the mini chocolate chips.

Scoop out 1 tablespoon of dough and roll into balls. Roll each ball in granulated sugar for a slightly crisp exterior. Place cookie dough balls on the prepared baking sheet, spaced about 2 inches apart. Bake for 9–11 minutes, until edges are set and centers look slightly soft.

Let cookies cool on the pan for 2 minutes, then transfer to a wire rack to cool completely. In a separate bowl, beat the softened butter until fluffy. Gradually mix in the powdered sugar, alternating with the milk. Add vanilla extract and a pinch of salt. Beat until light and creamy.

Pair cookies of similar size. Pipe or spread a generous amount of filling onto the flat side of one cookie. Top with another cookie to form a sandwich. Enjoy immediately or refrigerate for 15 minutes for a firmer filling.