Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan.

Melt 1 12-ounce package of chocolate chips in a saucepan with 2/3 cup of butter over very low heat, stirring constantly, until smooth. Watch carefully to avoid scorching; Remove from heat, and scrape into a large bowl. Beat in the sugar, a couple of tablespoons at a time, then beat in the eggs 2 at a time until the mixture is well combined. Stir in flour and salt until blended, then stir in the vanilla extract. Gently fold in 1 more 12-ounce package of chocolate chips and the pecans. Spread batter into the prepared baking pan.

Bake in the preheated oven until a toothpick inserted into the center of the brownies comes out with moist crumbs, about 35 minutes. Remove from oven and cool completely.

Mix the cream and 1/4 cup of butter together in a saucepan over low heat until the butter has melted, stirring constantly; pour in remaining 1 12-ounce bag of chocolate chips and stir to melt completely. Pour the mixture into a mixing bowl, and beat on Medium speed with an electric mixer until combined, about 1 minute; chill the frosting thoroughly, stirring occasionally, about 45 minutes to 1 hour. Mixture should be the consistency of thick cream; beat the frosting again with electric mixer until it forms stiff peaks, 3 to 5 minutes. Spread frosting on cooled brownies.