Preheat your oven to 350°F (175°C) and grease a 9x9-inch baking pan or line it with parchment paper.

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy, about 3-5 minutes. Add in the vanilla extract and mix well.

Gradually add the gluten-free flour blend and salt to the butter mixture, stirring until a soft dough forms. Press the dough evenly into the bottom of the prepared baking pan.

Bake the crust in the preheated oven for 20-25 minutes until lightly golden. Let it cool slightly.

In a medium saucepan, heat the sweetened condensed milk over medium heat, stirring continuously for 10-15 minutes until it thickens and turns a light caramel color. Pour the caramel over the cooled shortbread base and spread it evenly.

In a microwave-safe bowl, combine the chocolate chips, cocoa powder, and 1 tablespoon of butter. Microwave in 30-second intervals, stirring until melted and smooth. Pour the chocolate mixture over the caramel layer and spread it evenly.

Refrigerate the assembled Millionaire's Shortbread for at least 120 minutes to allow the layers to firm up. Once set, cut into squares and enjoy.