Melt the butter slowly in a small saucepan over medium-low heat until the milk solids have separated from the butterfat and collected on the bottom of the pan.
Remove the pan from heat, let the butter settle for 10 minutes, then carefully skim the foam from the surface with a spoon. Slowly pour the clear butterfat into a bowl, leaving all the milk solids behind in the saucepan.
In a small saucepan, combine the sugar and water, and place over medium heat, occasionally stirring, until sugar dissolves. Then add honey, salt, and vanilla.
Once boiling, reduce heat to low and simmer for about 20-25 minutes until slightly thickened. Remove the syrup from heat, let cool for about 10 minutes, and add lemon juice. Transfer to a measuring or pouring cup and set aside to cool completely before using.
Pulse pistachios and walnut in a food processor until coarsely chopped. Transfer to a bowl, then add cinnamon and mix until thoroughly combined and coarsely chopped.
Preheat the oven to 300β/150β.
Brush a 13x9-inch baking pan with clarified butter. Unwrap and unfold phyllo on a large cutting board and smooth out with hands to flatten. Using the pan as a guide, adjust the size of the phyllo by cutting off excess to fit perfectly into the pan.
Cover phyllo with plastic wrap, then top with a damp kitchen towel to prevent dough from drying and breaking while you work.
Place one sheet of phyllo in the bottom of the prepared pan and brush the top with the melted butter until completely coated. Layer 9 more phyllo sheets into the pan, brushing each sheet with more butter. You should now have a total of 10 layers of phyllo.
Top the dough with one cup of the ground nut mixture and spread evenly over the phyllo.
Layer 6 more sheets of phyllo into the pan, brushing each layer with more melted butter, then top with another cup of nuts. Repeat with six more phyllo sheets, butter, and the remaining cup of pistachios and walnut.
Layer the remaining ten phyllo sheets into the pan using the nicest, most intact sheets, brushing each layer, except the final layer, with more melted butter. Working from the center outward, use the palm of your hands to compress the layers and gently press out any air pockets.
Using a sharp knife, cut the baklava into about 24 diamonds: 4 cuts vertically and about 6 cuts diagonally. You can get up to 36 smaller diamonds depending on the size of the pan. Brush top of baklava with the remaining melted butter.
Bake the baklava until golden and crisp, about 90 to 100 minutes, rotating the pan halfway through baking.
Remove the baklava from oven, and immediately pour the cooled syrup over cut lines until about ΒΌ cup remains. Drizzle remaining syrup over the surface. Garnish center of each piece with pinch of ground pistachios.
Cool baklava to room temperature for about 2 hours, then cover with foil and let stand at least 8 hours before serving. Cut through the earlier marked pieces and serve.