Coat the bottom and sides of an 8-by-8-inch baking dish with nonstick spray.
In a large bowl, stir to combine the confectioners' sugar, graham cracker crumbs, peanut butter, 1/8 teaspoon salt, and melted butter. Press the mixture into the prepared baking dish and set aside.
Place the chocolate and 1/8 teaspoon salt in a medium bowl. Heat the cream in a small saucepan until bubbles start to form around the edges. Pour the cream over the chocolate, let sit for 1 minute and then whisk until completely melted and smooth.
Pour the ganache over the peanut butter mixture and refrigerate until the chocolate is cooled and set, at least 45 minutes and up to overnight.
Sprinkle evenly with the flaky sea salt. Cut into squares to serve.