Step 1

Brush a 8-by-8-inch baking dish with butter, and line with parchment paper, leaving a 2-inch overhang on long sides. Brush parchment with butter.

Step 2

Heat 1/2 cup condensed milk, 4 tablespoons butter, brown sugar, and a pinch of salt in a double boiler or a heatproof bowl set over a pan of simmering water, stirring, until melted, about 4 minutes. Remove from heat. Mix in peanut butter and 1 1/2 cup confectioners' sugar. Press peanut butter mixture into the bottom of prepared baking dish, smoothing top with an offset spatula.

Step 3

Melt chocolate in a double boiler or a heatproof bowl set over a pan of simmering water, stirring, until melted, about 3 minutes. Remove from heat, stir in remaining 3/4 cup condensed milk and a pinch of salt, until melted and combined. Mix in remaining 3/4 cup confectioners' sugar. Spread chocolate mixture over peanut butter layer, smoothing top with an offset spatula. Refrigerate until firm, about 2 hours.

Step 4

Remove fudge from pan using parchment to lift, and transfer to a cutting board. Peel off parchment. Using a warm knife, cut crosswise into eight 1-inch-wide strips; cut each strip into 8 pieces.