Position a rack in the center of your oven and preheat to 350. Spray an 8x8 pan with non-stick cooking spray. Line the pan with foil or parchment, leaving an overhang on 2 sides of the pan.
In a double-boiler or a heat-proof pan, combine butter and chocolate. Stir constantly, until mixture is melted and smooth. Remove from heat and stir in sugar and salt.
Whisk in your eggs, 1 at a time, and then whisk in extract and vanilla bean paste/powder. Gently fold in your flour with a spatula. When just a few streaks of flour remain, fold in the remaining 2 ounces of chocolate. Pour batter into prepared pan—it will be thick.
Dollop lemon curd on top of your mixture by tablespoonfuls. Using a knife, swirl the curd into your batter. Bake for 25 to 28 minutes, or until a toothpick inserted in the center comes out clean.
Transfer pan to a wire rack and allow to cool completely. Using the foil/parchment overhang, lift out the entire block and cut into 12 squares.