Break your chocolate into smaller pieces and melt it using au bain marie. Set aside to cool slightly.
Coarsely chop the nuts and set aside.
Preheat the oven to 350°F (180°C). Lightly butter a baking pan (I used a 25 x 25 cm square one) or line with foil and butter the foil.
Combine the flour, baking powder, salt and cinnamon in a bowl. Set aside. In a large mixing bowl, beat the butter until creamy on low speed. Beat in the sugar in four additions then beat in the eggs one at a time, adding the vanilla with the last egg. Scrape down the sides and beat until creamy and whipped up a bit.
Beat in the flour mixture in about 3 or 4 additions. Fold in the chopped nuts while scraping down the sides of the bowl and blending in the last of the flour.
Divide the batter in half and - using a spoon - place the vanilla batter in mounds into your baking pan. Beat or stir the cooled, melted chocolate into the remaining batter, blending well, then drop the chocolate batter in mounds in between and around the vanilla mounds of batter in the baking pan.
Swirl the batters together with a knife to marble. Bake… this could take anywhere from 40 minutes to an hour depending upon the size of pan used (and depth) and your oven. When baked, the top of the brownies should be glossy and crispy, the center set; a tester stuck into the center should come out mostly clean.