Position a rack in the lower third of the oven and preheat to 325 degrees . Line a 9x13 inch metal baking pan with foil, leaving about 4 inches of overhang on each side, then grease the foil.
In a medium saucepan, melt the butter and chocolate over medium-low heat, stirring until smooth. Remove from the heat; let cool for 10 minutes. Whisk in the sugar, then whisk in the eggs 1 at a time; stir in the vanilla. Stir in the flour until blended.
Spread the batter into the prepared pan and sprinkle evenly with the walnuts. Bake until the surface is set, 20-22 minutes. Immediately sprinkle sea salt over the hot brownies. Let cool completely in the pan, at least 30 minutes.
Lift the brownies out of the pan and cut in half crosswise. Wrap the 2 halves in foil and freeze for 30 minutes.
Place 1 brownie half, walnut side down, on a work surface. Spread fudge sauce or chocolate sauce evenly on top. Return, uncovered, to the freezer.
Let ice cream soften until spreadable. Place the brownie half, fudge side up, on a work surface. Spread the softened ice cream evenly on top, then sandwich with the other brownie half, walnut side up. Freeze until set, at least 30 minutes. Cut into 16 squares. Serve, or wrap and freeze for up to 2 weeks.