Line an 8 x 6 quarter sheet pan with parchment paper and press it against the sides.

In a mixing bowl, beat softened unsalted butter on medium speed for about 2 minutes until smooth.

Gradually add powdered sugar on low speed and mix until fully combined, about 3-4 minutes.

Add blueberry flavor paste and a pinch of salt, mixing on low speed until evenly distributed.

Add whole milk one tablespoon at a time until desired consistency is reached, about 4-5 tablespoons.

Spread the buttercream mixture in the prepared pan and refrigerate for 15-20 minutes until firm.

Remove from the fridge and cut into 1-inch squares using a sharp knife.

Allow squares to air dry at room temperature for 12-24 hours.

Melt dark chocolate in a microwave-safe bowl in 30-second intervals until smooth.

Dip each buttercream square into melted chocolate, ensuring even coating, then place on a tray.

Let the chocolates set at room temperature for about 30 minutes or chill for a shiny finish.

Store in an airtight container in a cool, dry place.