2/3 cup all-purpose flour plus more for the pan
8 ounces coarsely chopped bittersweet or semisweet chocolate
at least 70 percent cacao
½ teaspoon baking powder
¼ teaspoon sea salt
2 large eggs
2 teaspoons vanilla extract
½ cup sugar
Pre-heat oven to 350 degrees.
Coat an 8-inch-square baking pan with butter. Add a spoonful of flour tilt to coat and tap out any excess; set aside.
In a double boiler or a large heatproof bowl placed over a pan of simmering water combine the chocolate and butter. Heat stirring occasionally until the chocolate melts. Remove from heat; let cool.
Meanwhile combine the flour baking powder and salt in a medium bowl; set aside.
Whisk the eggs and vanilla extract in a large bowl until foamy.
Add the sugar and whisk until light and frothy about 1 minute.
Slowly stir in the cooled chocolate mixture.
Add the flour mixture in 2 batches whisking until incorporated after each addition.
Scrape the batter into the prepared pan.
Bake until a skewer or toothpick inserted 1 inch from the edge of the pan comes out clean 20 to 25 minutes (a bit of moist batter should cling to the skewer).
Transfer the pan to a rack to cool for 10 minutes before slicing.