To make the brownies, preheat oven to 325°F and butter a 15-1/2 x 10-1/2 x 2-inch baking pan. In a small saucepan, melt the butter and chocolate over low heat. Let cool. Onto a piece of waxed paper, sift the flour and salt. Toss with the nuts.
In a large bowl, with a mixer at high speed, beat the eggs just until foamy. Add the sugar, then the cooled chocolate mixture and vanilla, and beat just until blended. Stir in the flour mixture just until it disappears, then spoon into the prepared pan. Bake for 35 minutes, or just until set and a toothpick inserted in the center comes out almost clean. Let cool completely in the pan before frosting.
To make the frosting, heat the chocolate, butter, and milk in a small saucepan over low heat until chocolate melts, stirring frequently. Cool to lukewarm and pour into a medium bowl. Stir in sugar and vanilla. Using a mixer, beat at high speed for about 10 minutes, or until thick enough to spread. Frost completely cooled brownies in pan.
Refrigerate until ready to serve, then cut into 1-inch chunks and remove from pan.