Preheat oven to 325°F (170°C). Spray an 8-inch square baking pan with cooking spray. Line pan with parchment paper, letting excess extend over sides of pan.

In the top of a double boiler, combine chocolate and butter. Cook over simmering water, stirring occasionally, until melted and smooth. Turn off heat, and whisk in sugars until well combined. (Mixture will not be completely smooth.) Remove from heat, and let cool slightly, 3 to 5 minutes.

In a medium bowl, whisk together flour, cocoa, salt, and espresso powder.

Slowly add eggs to chocolate mixture, whisking until combined. Whisk in vanilla. Fold in flour mixture just until combined. Place 1 cup (278 grams) batter in a pastry bag fitted with a ½-inch round piping tip (Ateco #804). Spread remaining batter (about 3 cups or 835 grams) in prepared pan.

Place Lemon Curd in another pastry bag fitted with a ½-inch round piping tip (Ateco #804). Pipe a border of batter around edges of pan. Starting with curd, pipe alternating lines of curd and batter (8 lines of curd, 7 lines of batter). Starting in top left corner about 1 inch down and perpendicular to piped lines, drag a wooden pick from one side to the other. Pick up wooden pick, move 1 inch down, and drag through piped lines in opposite direction. Repeat until entire pan is complete.

Bake until a wooden pick inserted in center comes out with a few moist crumbs, 40 to 45 minutes. Let cool completely in pan. Using excess parchment as handles, remove from pan, and cut into bars. Store in an airtight container for up to 6 days. Let come to room temperature before serving. Refrigerate in an airtight container for up to 3 days.