Preheat oven to 350ºF (180ºC).
Butter or spray an 8-inch square baking pan. Line it with parchment paper for easier removal. See Notes below.
Melt ½ cup unsalted butter and 8 ½ ounces semisweet chocolate in a large bowl. I use the microwave, 20-30 seconds at a time, stirring well in between.
Add ¾ cup sugar and 3 eggs, integrating very well but not beating.
Add 1 cup hazelnuts, ground, ¼ cup rice flour, ¼ teaspoon salt and ½ teaspoon baking powder to the chocolate mixture and mix well.
Add 1 teaspoon vanilla extract and 1 tablespoon hazelnut liqueur and mix just until blended.
Spread the brownie mixture on the prepared pan and even out the top.
Bake for about 25 minutes, until it is slightly puffed and the center still jiggles a little. Don't overbake for the best results. A cake tester or toothpick should come out with moist crumbs.
Cool completely on a wire rack.
For easier cutting, put 30 minutes in the fridge. Make sure the unlined sides of the pan are not stuck, and lift the whole block with the help of the paper.
Cut into squares.
You can keep them at room temperature for a few days, in the fridge for a week, or freeze them for a month.