Melt the butter and chocolate squares in a double boiler, stirring every 30 seconds, until completely melted. Let cool slightly before stirring in the sugar. Beat in the eggs and vanilla until well combined. Sift in the flour and cinnamon and mix. Fold in 1 cup chopped walnuts and 1/2 chopped pistachios. Set brownie batter aside.
For the brownie batter:
In a large bowl set over a pan of simmering water, melt the butter and chocolate together over a low heat, stirring constantly until melted.
Take off the heat and stir in the sugar, eggs and vanilla until smooth. Fold in the flour, salt, cinnamon, walnuts and 1/2 a cup of the pistachios.
Unroll the phyllo dough out on a work surface. Place a 9-inch spring form cake pan on the center of the pastry. Use a sharp knife to cut all the way around the cake tin to make circle-shaped pieces of dough. Keep the phyllo covered with a damp cloth while you are assembling the baklava.
Brush the base and sides of the springform pan with melted butter. Place one circle of phyllo dough at the bottom of the cake pan. Brush with melted butter. Repeat this so you have a total of 7 layers of phyllo in the cake pan, each layer having been brushed with butter.
Spread half of the brownie batter evenly over the phyllo dough in the cake pan.
Layer 7 more sheets of phyllo dough over the brownie batter, brushing each with a layer of butter before laying on the next.
Spread the remaining brownie batter over the phyllo.
Top this with the remaining phyllo sheets, brushing each layer with melted butter before laying on the next. This will be quite a thick section of phyllo layers!
Chill the whole thing for 2 hours, so the brownie batter has firmed up which will make it easier to cut.
Once it has been chilled, preheat the oven to 350 degrees F.
Using a sharp knife, cut the unbaked baklava into the chevron star pattern (as shown in the video) going all the way through the top layers of phyllo.
Bake for 50-60 minutes, until a skewer inserted into the center of the brownie, comes out with some moist crumbs and a bit of batter clinging to it (it shouldn’t be completely gooey in the middle, but it also shouldn’t be completely cooked through or the brownie will be dry).
Remove from the oven and pour the honey over the top of the brownie. Top with the remaining chopped pistachios.
Let cool completely, for 2-3 hours, before removing from the spring form pan and cutting into pieces.