Prepare the Pan: Line an 8 x 6 quarter sheet pan with parchment paper, ensuring the paper hangs over the edges for easy removal later.

Cream the Butter: Beat the softened unsalted butter on medium speed until it becomes light and creamy, about 2 to 3 minutes.

Incorporate Powdered Sugar: Gradually add the powdered sugar into the creamed butter, mixing on low speed initially to prevent a sugar cloud.

Add Blueberry Flavor and Salt: Mix in the blueberry flavor paste and a pinch of salt, ensuring they blend evenly into your buttercream.

Adjust Consistency: Slowly add whole milk one tablespoon at a time, mixing well after each addition to achieve a smooth, pipeable consistency.

Chill the Buttercream: Cover the bowl with plastic wrap and chill it in the refrigerator for about 15 to 20 minutes.

Cut into Squares: Spread the buttercream evenly in the prepared pan and cut into 1-inch squares.

Allow to Dry: Let the cut buttercream squares dry at room temperature for 12 to 24 hours.

Melt the Chocolate: Melt the dark chocolate in a double boiler over low heat, stirring gently to prevent scorching.

Dip the Chocolates: Dip each buttercream square into the melted chocolate, then place on a parchment-lined tray.

Let Chocolate Set: Allow the dipped chocolates to cool at room temperature until the chocolate coating is fully set, about 30 minutes to 1 hour.

Storage: Transfer your chocolates to an airtight container for storage.