• Preheat oven to 350F. Butter a 9×9 baking pan and line with parchment paper, leaving overhang if you’d like to remove the bars for neater slicing.

make the crust

• Whisk the flour, cocoa powder, powdered sugar, and salt together in a bowl. Stir in the melted butter and vanilla until the mixture is evenly moistened and holds together when squeezed.
• Press the crumbly mixture firmly into the 9x9 pan and bake for 18 minutes.

make the filling

• Whisk the filling ingredients together until smooth and the eggs are completely incorporated.
• When the crust comes out of the oven, gently pour the filling over the top. One trick is to pour it over the back of a large spoon so it lands softly on the delicate crust. A few chocolate crumbs might come loose and float in the filling, that's not a problem.
• Continue baking for 22–25 minutes, or until just set (the center might have a slight wobble.)
• Let the bars cool at room temperature, then chill before slicing. Dust with powdered sugar before serving.
• If you’ve used parchment paper, lift the chilled bars out of the pan for easier slicing. Use a long sharp knife and wipe it clean between cuts. Store the bars in the refrigerator.