Brown the butter. In a small saucepan over medium heat, melt the butter. Continue cooking, swirling occasionally, until the butter turns golden brown and smells nutty. Remove from heat and let cool completely.
Whip the ricotta. Add the ricotta to a food processor or high-speed blender and process until completely smooth and creamy. Add the powdered sugar, vanilla extract, sea salt, and cooled brown butter. Blend again until light and fluffy.
Prepare the espresso soak. In a shallow bowl, add the cooled espresso.
Layer the tiramisu. Quickly dip each ladyfinger into the espresso (do not soak, just a brief dip). Arrange a single layer of dipped lady fingers in your serving dish. Spread half of the ricotta cream evenly over the cookies. Sprinkle a generous layer of finely chopped dark chocolate over the cream. Repeat with another layer of dipped ladyfingers, the remaining ricotta cream, and finish with another light sprinkle of chopped chocolate if desired.
Cover and refrigerate for at least 4 hours, preferably overnight, to allow the layers to set.
Just before serving, dust generously with unsweetened cocoa powder. Slice and serve chilled.
