Preheat the oven to 350°F (175°C).
Line a 9x9 inch (23x23 cm) square baking pan with parchment paper, making sure the paper hangs over the edges for easy removal.
In a microwave-safe bowl, combine the chopped chocolate with the melted butter. Heat in 20-30 second intervals, stirring between each, until the chocolate is completely melted and the mixture is smooth.
1 cup chopped chocolate, ½ cup melted butter
Add the sugar, eggs, and vanilla extract to the chocolate and butter mixture. Stir well until all ingredients are fully combined and the mixture is smooth.
1 cup granulated sugar, 2 eggs, 1 teaspoon vanilla extract
In the same bowl, sift in the flour, baking soda, and salt. Mix until all the ingredients are well incorporated and the batter is smooth.
1 ¼ cups all-purpose flour, ½ teaspoon baking soda, ½ teaspoon salt
Pour the batter into the prepared pan and spread it evenly with a spatula. Drop spoonfuls of Biscoff butter over the brownie batter. Use a toothpick or the tip of a knife to swirl the Biscoff butter throughout the batter, creating a marbled effect.
1 cup Biscoff butter
Bake in the preheated oven for about 30 minutes, or until a toothpick inserted into the center comes out with moist crumbs but not raw batter.
Let the brownies cool completely in the pan before cutting into squares.
Top the brownies with a sprinkle of flaky sea salt or Maldon salt before serving.