Preheat the Oven

Twenty minutes or longer before baking, set an oven rack in the middle of the oven and preheat the oven to 325°F/160°C (300°F/150°C if using a Pyrex or dark pan).

Break and Toast the Pecans

Break the pecans into medium pieces. Spread them evenly on a baking sheet and bake for about 10 minutes to enhance their flavor. Stir once or twice to ensure even toasting and avoid overbrowning. Cool completely.

Melt the Butter and Chocolate

In a double boiler over hot, not simmering, water (do not let the bottom of the container touch the water), melt the butter and chocolate, stirring often. Scrape the melted chocolate mixture into the bowl of a stand mixer or large mixing bowl.

Make the Batter

Stand Mixer Method In the bowl of a stand mixer fitted with the flat beater, beat the cocoa and sugar into the melted chocolate mixture on medium speed until incorporated. Scrape down the sides of the bowl. Beat in the eggs and vanilla for about 30 seconds. The mixture will become thick and glossy. Beat in the cream cheese until only small bits remain. Add the flour and salt and mix on low speed just until the flour is moistened, about 10 seconds. Add the pecans and mix for 3 seconds.

Hand Method In the bowl containing the melted chocolate mixture, whisk in the cocoa and then the sugar until incorporated. Whisk in the eggs and vanilla until the mixture becomes thick and glossy. With a blending fork or wooden spoon, stir in the cream cheese until only small bits remain. Add the flour and salt and stir just until the flour is moistened. Stir in the pecans.

Fill the Pan

Scrape the batter into the prepared pan and smooth the surface evenly with an offset spatula.

Bake the Brownie

Bake for 25 to 35 minutes, or until the batter has set only to 1 inch from the edge and a toothpick inserted 1 inch from the edge comes out clean. (An instant-read thermometer inserted into the center should read about 190°F/88°C.)

Cool the Brownie

Let the brownie cool in the pan on a wire rack until completely cool to the touch, about 2 hours. To speed cooling, the pan can be set in the refrigerator. Run a small metal spatula between the pan and the parchment to ensure that no batter has leaked through and stuck to the sides. Gently grasp the ends of the parchment and lift it up a little to make sure the brownie will release from the pan. If desired, lift the brownie out of the pan, use a long serrated knife to level the top, then return the brownie to the pan.

Make the White Chocolate Custard Base

See recipe.

Frost the Brownie

Scrape the white chocolate buttercream onto the brownie and, using a small offset spatula, evenly frost it with about 1½ cups/9.7 ounces/275 grams of the white chocolate buttercream. (Refrigerate or freeze the leftover buttercream for future use.) Refrigerate the frosted brownie for a minimum of 1 hour to firm the buttercream before applying the ganache glaze.

Make the Dark Chocolate Ganache Glaze

See recipe.

Unmold and Cut the Brownies

Grasp two of the opposite overhanging edges of the parchment and lift out the brownie. Set it on a countertop. If desired, for perfectly squared off corners, use a serrated knife to trim the sides of the brownie. Use a thin bladed knife, run under warm water and wiped dry between each cut, to cut the brownie into squares. To retain the pristine white of the buttercream, cut straight down and then pull the knife straight out. If desired, cut each square in half diagonally to make triangles.