Dough
Adjust the oven rack to the middle position and preheat oven to 350 °F (175 °C).

In the bowl of a food processor, add flour, sugar, cornstarch, salt and pistachios. Run the processor until the pistachios are ground up. Add the butter and pulse on and off until the dough comes together.

On a lightly floured surface, roll the dough out into a 13 ¾ inch x 4 ⅓ inch (35 cm x 11 cm) rectangle, ¼ inch (6.5 mm) thick. In a 13 ¾ inch x 4 ⅓ inch  (35cm x 11 cm) tart pan, place the dough and press along the edges to form the dough to the pan.

Bake until lightly browned, about 20-25 minutes. Allow to cool.

Pistachio Topping
Put the sugar, honey, orange blossom water, egg white, vanilla, baking powder, and salt in a medium bowl. Stir with a rubber spatula to thoroughly combine. Add the pistachios and stir to coat the nuts in the syrup.

Macaroon Filling: 
In the bowl of a food processor, grind the nuts with half of the sugar, until it is a fine meal consistency. Add the flour, remaining sugar, and pulse to combine. Pour in two-thirds of the egg whites and process until it is a smooth paste. Add the remaining egg whites and pulse on and off until completely combined. Transfer the nut mixture into a large bowl.

In a small saucepan, over medium high heat, begin to melt the butter. Using a small paring knife, split the vanilla bean in half lengthwise and scrape out the seeds with the back of the knife. Add the scrapped seeds and the bean to the butter. After a few minutes the butter will begin to foam and darken. Swirl the pan to promote even browning taking care it does not burn. Continue to cook about 5-7 more minutes until the bubbles subside and the liquid is dark brown with a nutty and toasty aroma. Remove the vanilla bean.

Slowly pour the brown butter into the nut mixture, whisking continuously. Pour the mixture into the tart pan over the dough. Bake for 8 minutes.

Remove the tart pan from the oven and quickly distribute the nut topping on top. Return the tart back to the oven and finish baking for about 15-20 minutes until the nut topping is golden brown, rotating the tart halfway to ensure even browning. Remove the tart form the oven and set it aside to cool to room temperature.

To serve, pour the remaining powdered sugar for dusting into a fine mesh sieve and tap the sieve over the tart to dust it lightly. Gently push the removeable bottom to lift the tart out. Using a long offset spatula, gently slide the tart from the metal bottom onto a long platter or wooden board.

Equipment: Food processor, rolling pin, rubber spatula, 13 ¾ inch x 4 ⅓ inch (35 cm x 11 cm) rectangular tart pan with removable bottom, medium bowl, paring knife, whisk, fine mesh sieve, 10 inch offset spatula, platter or wooden board.