Arrange an oven rack to the middle position and preheat the oven, with a rack placed in the center, to 375°F. Butter an 8-inch round cake pan and line the bottom with parchment; also butter the parchment.
Melt the chocolate and butter in a heavy saucepan over medium low heat, stirring constantly, 3 to 5 minutes.
When the butter and chocolate are melted, remove from the heat. Add the sugar and whisk to combine. Add the eggs, one at a time and whisking well to incorporate each egg before adding the next one. The batter will look grainy but shiny until the last egg goes in, and then the batter will be smooth and silky.
Add the vanilla, espresso powder, salt, and cocoa powder and whisk to combine.
Pour the batter into the prepared pan. Bake the cake for 24 to 25 minutes, or until the top is crackly and the center no longer looks shiny. Cool the cake for about 15 minutes. Invert the cake out a wire cooling rack so the bottom is facing up. Set the rack over a sheet of parchment.
Make the glaze. In a heavy saucepan over medium heat, or in a medium heatproof bowl in the microwave, heat the cream until warmed through. Add the chocolate chips and set aside to allow the chips to melt, 5 minutes. Stir to thoroughly combine the chocolate and cream, until smooth. Add the corn syrup and stir to combine. Reserve 2 tablespoons of the ganache and set aside.
Immediately pour the glaze over the cake, allowing it to run off to fully coat the sides of the cake and the top evenly. Use a large offset spatula to spread the glaze if needed. Allow the glaze to set for 10 minutes, then garnish with the pistachios and rose petals, if using. Chill the cake on the wire rack in the refrigerator for 2 hours.
On the serving platter, place a dollop of reserved ganache in the center to anchor the cake. Carefully transfer the chilled cake to the serving platter. To cleanly slice the cake, run the blade of a sharp chef's knife under hot water and dry it off. Slice into wedges, wiping down the knife with a paper towel after each cut, and serve.