Preheat your oven to 350°F (175°C).
In a medium saucepan over low heat, melt the 4 tablespoons of unsalted butter, 2 tablespoons of oil, and 1 cup of dark chocolate pieces. Stir frequently until smooth and well combined. Remove from heat and let it cool slightly.
In a separate mixing bowl, whisk together the 2/3 cup of sugar and the 2 large eggs until the mixture is light and fluffy.
Once the chocolate mixture has cooled slightly, add it to the sugar and egg mixture. Stir until fully incorporated.
In another bowl, combine the pinch of salt, 1 tablespoon of cocoa powder, and 1/4 cup of gluten-free oat flour. Gradually add this dry mixture to the wet ingredients, folding gently until just combined.
Gently fold in the 1/2 cup of milk chocolate pieces, 1/2 cup of chopped hazelnuts, and 1/3 cup of raisins.
Line your 9x9-inch baking pan with parchment paper and pour in the brownie batter. Spread it evenly and bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Allow the brownies to cool in the pan for at least 10 minutes before lifting them out using the parchment paper. Let them cool completely on a wire rack before cutting into squares.