For the best results, line a square pan with parchment paper and very lightly butter the parchment.
In a medium saucepan, combine the peanut butter and butter. Heat over medium-low heat, stirring frequently, until fully melted and smooth. Continue to heat and stir until larger bubbles appear just at the outside edge of your pot. Remove from heat.
1 cup (264 grams) creamy peanut butter, 1 cup (2 sticks or 227 grams) unsalted butter
Stir in the vanilla and salt and then add the powdered sugar. Mix until you no longer see any traces of sugar.
1 teaspoon vanilla extract, ¼ teaspoon salt, 3½ cups (420 grams) powdered sugar
Add the peanut butter fudge layer to the prepared pan, pressing down. You do not need to get it perfectly smooth and level. Set the pan aside.
While the peanut butter fudge is still warm, make the ganache. Place the chopped chocolate in a heat-proof bowl, and heat the cream in a small pan over low heat. When the cream is just barely bubbling, add it to the chocolate. Let it sit for a moment and then stir together until combined and shiny.
8 ounces (227 grams) bittersweet chocolate, 4 ounces (120 milliliters) heavy cream
Pour the ganache over the peanut butter fudge and use a knife or off-set spatula to smooth it over the top. Chill your peanut butter and chocolate fudge in the refrigerator for at least three hours before cutting.