How to Make It
Lightly grease an 8-inch-square baking dish and line it with parchment paper, leaving a 2-inch overhang on 2 sides.
In a medium saucepan, melt the stick of butter over moderate heat. Add the brown sugar and half-and-half and bring to a boil, stirring occasionally, then continue to boil for 2 minutes without stirring. Remove from the heat and whisk in the peanut butter, salt and vanilla until well combined.
Put the confectioners’ sugar in a large bowl. Pour the hot peanut butter mixture over the sugar and whisk until no streaks remain. Scrape the fudge into the prepared baking dish and smooth the top. Fold the parchment over the warm fudge, press it directly onto the surface and refrigerate until completely chilled, about 2 hours.
Using the parchment, lift the fudge out of the baking dish, garnish with flaky salt and cut into 1-inch squares.