• Heat oven to 350ºF. Butter the bottom and sides of a 9x9x2 baking pan, and line the bottom with parchment or aluminum foil.
• Melt the butter and chocolate in a double boiler or a medium heatproof bowl set over a pot of boiling water, stirring occasionally with a small whisk until smooth. Remove from heat and let cool for 2 to 3 minutes. If you used a double boiler, transfer chocolate to a medium bowl.
• Stir both sugars into the chocolate mixture until fully combined. Using a rubber spatula or a wooden spoon, mix in the eggs one at a time. Add the flour, vanilla paste, and salt, and stir until thoroughly combined.
• Spread the batter into the prepared baking pan. Bake for 25 to 30 minutes, until a crust forms and the center appears dry. Check the brownies with a toothpick- small crumbs should adhere (do not bake until a tooth pick inserted comes out totally clean). Let the brownies cool in pan for at least 10 minutes on a cooling rack before cutting with a sharp knife
Once cooled, the brownies can be stored in an airtight container for up to 5 days; or leave the brownies, uncut,in the pan, wrap in double layer of plastic wrap, and freeze up to 2 months.