Line an 8x6 quarter sheet pan with parchment paper, ensuring it covers the bottom and sides completely.

In a mixing bowl, add 1 cup of unsalted butter at room temperature and beat on medium speed for 2–3 minutes until smooth.

Gradually add 4 cups of powdered sugar, mixing on low speed, then increase to medium for 2 minutes until combined and smooth.

Add 2 tablespoons of blueberry flavor paste and a pinch of salt, mixing on medium speed for 1–2 minutes until evenly distributed.

Slowly add whole milk 1 tablespoon at a time, mixing until the buttercream reaches desired smoothness—about 3–4 minutes.

Spread the blueberry buttercream filling evenly into the prepared pan and chill in the refrigerator for 15–20 minutes.

Once chilled, cut the buttercream into 1x1 inch squares and transfer to a parchment-lined tray.

Melt 8 ounces of dark chocolate in a microwave-safe bowl in 30-second intervals, stirring until smooth.

Dip each buttercream square into the melted chocolate, allowing excess to drip before placing on a parchment-lined tray.

Let the chocolates sit at room temperature for 30–60 minutes until the chocolate is completely set.