Preheat your oven to 350°F (175°C) and line a 9x9 inch baking pan with parchment paper. To caramelize the white chocolate, place the chopped white chocolate in a heatproof bowl over a double boiler or microwave. Heat until melted and smooth, stirring frequently until golden brown, usually about 10-15 minutes on the stove or 2-3 minutes in the microwave.
Melt the unsalted butter and dark chocolate together until smooth in a separate bowl. Allow it to cool slightly, then whisk in the granulated sugar. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract and buttermilk.
In another bowl, whisk together the all-purpose flour, sifted cocoa powder, salt, and baking powder. Gradually add the dry mixture to the wet mixture, stirring gently until just combined.
Fold in the caramelized white chocolate. Pour half of the brownie batter into the prepared pan, drizzle half of the caramelized white chocolate over it, then add the remaining brownie batter and the rest of the caramelized white chocolate. Swirl together for a marbled effect.
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Let cool in the pan for at least 15 minutes before lifting out using the parchment paper.
Allow the brownies to cool completely on a wire rack before slicing into squares. Serve warm or at room temperature.
