Line an 8-inch square pan with parchment, leaving overhang for easy lifting.
Stir melted butter and 1 1/4 cups peanut butter until smooth. Mix in vanilla and salt.
Add powdered sugar gradually and fold until a soft dough forms with no dry spots.
Press peanut butter mixture firmly into the lined pan, smoothing the top.
Melt chocolate chips, condensed milk, and 2 tbsp peanut butter in a saucepan over low heat, stirring until glossy.
Spread the warm chocolate evenly over the peanut layer. Tap the pan to level.
Sprinkle with flaky salt if using. Chill until set, about 1–2 hours, then cut into small squares.