Preheat oven to 350°F. Grease or line a 9x12-inch pan with parchment paper.

In a large bowl, whisk together flour, cocoa powder, powdered sugar, and salt.

Cut in cold butter until crumbly.

Add the egg yolk and 2 tablespoons milk. Mix until dough just comes together. Add more milk as needed, 1 teaspoon at a time.

Press dough evenly into the bottom of the pan. Dock lightly with a fork.

Bake 12–14 minutes, until dry to the touch. Cool completely on a wire rack.

Melt butter and chocolate chips together in a saucepan over medium heat, stirring until smooth.

Pour into a large bowl, whisk in sugar, then add eggs one at a time, whisking until glossy.

In a separate bowl, whisk together flour, cocoa powder, salt, and espresso powder (if using).

Stir dry ingredients into the chocolate mixture until just combined.

Pour over the cooled crust and spread evenly.

Bake at 350°F for 18–22 minutes, until a toothpick comes out with moist crumbs. Don’t overbake.

Cool completely on a wire rack.

In a saucepan, whisk sugar and eggs constantly over medium heat until the mixture reaches 160°F and thickens enough to coat the back of a spoon.

Remove from heat and stir in melted chocolate and vanilla. Let cool to about 75°F.

In a stand mixer, beat softened butter until fluffy (2–3 minutes).

Gradually beat in the cooled chocolate mixture, then whip on high speed for 5 minutes until light and silky.

In a clean large bowl, whip heavy cream and powdered sugar until stiff peaks form.

Gently fold whipped cream into the chocolate mixture.

Spread evenly over the cooled brownie layer. Cover with plastic wrap and chill for at least 6 hours or overnight.

In a cold bowl, whip cream, powdered sugar, and vanilla until stiff peaks form.

Spread over the chilled French silk layer.

Garnish with chocolate curls, chopped pecans, or a dusting of cocoa powder if desired.