Preheat the oven to 350°F. Grease a 8x8 square pan and line with parchment or foil.
In a microwave-safe bowl, combine butter and chocolate and heat for 1 minute. Stir, then continue heating in 30-second increments until chocolate is nearly melted, then stir until smooth. (You can also use a saucepan set over low heat.)
Stir sugar into chocolate mixture. Add eggs, one at a time, then stir in salt and vanilla. Stir in flour until just incorporated, scraping the sides and bottom of the bowl, then fold in half the pecan pieces.
Pour into prepared pan and smooth to the edges. Place in the oven and bake for 20 minutes. Meanwhile, make the sauce.
To make the sauce, in a small saucepan combine butter and brown sugar over medium heat. Once sugar is fully melted, whisk in cream and salt. Bring to a boil and cook for about 5 minutes, whisking to prevent scorching. Remove from heat and whisk in vanilla, then set aside.
Remove brownies from oven and poke all over with a toothpick. Drizzle butterscotch sauce over the top and scatter on remaining pecans.
Return to oven for another 10-15 minutes, depending on how gooey you like your brownies. Let cool before serving.