How to Make It
Preheat the oven to 325°. Liberally grease a large rimmed baking sheet with baking spray. Line the sheet with parchment paper and liberally grease the paper.
In a large bowl, sift the granulated sugar, flour, cocoa powder, baking soda, baking powder and salt. In a medium bowl, whisk the eggs with the milk.
In a small saucepan, combine the butter, vanilla and espresso powder with 1/2 cup of water and bring to a simmer, stirring to dissolve the espresso. Drizzle the hot mixture over the dry ingredients. Add the egg mixture and whisk until smooth. Scrape the batter onto the prepared sheet and spread in an even layer. Bake for about 20 minutes, until set and a cake tester inserted in the center of the cake comes out clean (the cake will look wet). Transfer the sheet to a rack and let cool.
In a medium bowl, using a hand mixer, beat the butter with the confectioners’ sugar at medium speed until light, about 2 minutes. Beat in the peanut butter and salt, scraping down the side of the bowl as necessary. Beat in the milk and vanilla.
Line a large baking sheet with parchment paper and generously grease it with baking spray. Invert the cooled cake onto the prepared sheet. Peel the parchment paper off the cake; it will be sticky. Cut the cake in half lengthwise, then crosswise. Spread the filling all over the cake pieces, leaving a 1/4-inch border all around the sides. Roll up the cakes lengthwise (the parchment will come off easily) to form 4 logs. Refrigerate for 1 hour.
In a bowl, combine the chocolate, butter, corn syrup and salt. In a small saucepan, bring the cream to a simmer. Pour the hot cream over the chocolate mixture and whisk until smooth. Let cool.
Line another large baking sheet with parchment paper. Cut each cake log into 3 equal pieces. Working with 1 piece at a time and using a fork or skewer, dip the logs into the ganache to coat all over, letting the excess drip off. Transfer the cakes to the prepared sheet and sprinkle with flaky salt or peanuts. Freeze until firm, about 1 hour, before serving.