Step 1

Preheat oven to 350°F. Line a 9-inch square baking pan with parchment paper, leaving at least 2 inches of overhang on all sides. Lightly grease parchment paper with softened butter. Stir together flour, coconut sugar, desiccated coconut, and salt in a medium bowl. Stir in melted butter until well combined. Spoon mixture into prepared pan, and spread evenly. Firmly press mixture into an even layer using bottom of a dry measuring cup. Bake in preheated oven until light golden brown, 12 to 15 minutes. Transfer pan to a wire rack. Do not turn oven off.

Step 2

After about 10 minutes of pastry base bake time, make the tamarind caramel: Melt butter in a small saucepan over medium-low. Stir in condensed milk, coconut sugar, tamarind concentrate, and salt; cook, whisking constantly, until mixture is smooth and glossy and just begins to bubble, about 5 minutes.

Step 3

Pour hot caramel over pastry base in pan, and spread in an even layer. Return to oven, and bake at 350°F until caramel is bubbly and set in the center and edges are slightly puffed, 8 to 10 minutes. Transfer pan to a wire rack. Let cool completely, about 1 hour and 30 minutes.

Step 4

Fill a medium saucepan with water to a depth of 1 inch. Bring to a boil over medium-high. Reduce heat to medium-low to maintain a simmer. Combine chocolate and butter in a medium-size heatproof bowl, and set over pan of simmering water, ensuring bottom of bowl doesn't touch water. Cook, stirring often, until mixture is smooth and melted, 3 to 6 minutes.

Step 5

Pour topping over caramel on cooled pastry, and spread in an even layer; tap pan on counter to smooth surface. Loosely cover pan with plastic wrap, ensuring plastic wrap doesn't touch topping, and chill until topping is hardened, about 1 hour and 30 minutes.

Step 6

Using parchment paper overhang as handles, lift pastry bar from pan, and transfer to a cutting board. Cut evenly into 36 (1 1/2-inch) squares, wiping knife clean with a damp towel after each cut. Serve at room temperature.