Make the Brownie Cake: Preheat oven to 350 degrees F (180 degrees C). Lightly coat a 9-inch (23 centimeter) springform pan with nonstick cooking spray.
In a medium saucepan over medium heat, melt the butter. Remove the pan from the heat, add the dark chocolate, and let sit for about 30 seconds, then whisk it all together until smooth and glossy. Add the brown sugar, vanilla, and espresso powder and whisk until smooth. Whisk in the eggs, 1 at a time, then sprinkle in the flour, baking powder, and salt. Use a rubber spatula to fold it all together until there is no flour visible.
Spread the brownie mixture in the prepared springform pan and bake for 20 to 25 minutes, or until the cake has puffed slightly and the top is shiny and just barely cracked. Transfer to a wire rack to cool completely before removing the sides of the springform pan. In the meantime, make the speculoos cookie butter frosting.
Make the Speculoos Cookie Butter Frosting: In a large bowl, using an electric mixer on medium speed, mix together the speculoos cookie butter and unsalted butter for a full minute so it's light and fluffy. Add the powdered sugar, salt, and cream and beat for another minute, or until the mixture is fluffy and spreadable.
Spread the frosting on the cake, garnish with chocolate shavings, and cut into wedges to serve. Leftovers can be stored, covered, at room temperature for up to 3 days or in the refrigerator for up to 5 days.