1-Start by preparing the Pâté Sucrée for the crust, which sets the foundation for this brownie tart. Cream butter and sugar together until light and fluffy, taking about 3-5 minutes, before adding salt, vanilla extract, and beaten egg. Mix until combined, then gradually add flour in two parts, stirring just until the dough forms without overmixing. Shape it into a ball, flatten into a disc, wrap it tightly, and chill for at least 1 hour or overnight this step helps it hold up well.

2-Once chilled, preheat your oven to 350°F (175°C) and place a baking sheet on the bottom rack. Grease a 9-inch tart pan, press the dough evenly into it, trim any excess, and poke holes with a fork. Freeze the crust for 10 to 30 minutes to firm it up, then line it with parchment paper and pie weights before baking on the sheet for 20 minutes. Remove the weights and paper, and bake for another 5 to 10 minutes until set and cooled.

3-For the filling, melt butter with dark and bittersweet chocolates in short bursts, stirring until smooth, then mix in vanilla extract and let it cool slightly. In another bowl, whisk eggs, brown sugar, and granulated sugar for 2-3 minutes until combined. Slowly add the chocolate mixture to the eggs, stirring gently, and sift in cocoa powder, flour, and corn starch, folding until just mixed. Pour into the baked crust, smooth the top, and sprinkle with flaky salt if you like. Bake for 40 to 45 minutes until the edges puff up and the center doesn’t jiggle. Let it cool fully before slicing the whole process takes about 20 minutes of prep, 70-75 minutes of cooking, and that chilling time.