Preheat oven to 350°F. Grease a 9-inch round cake pan with cooking spray and dust with flour, set aside.

To make the cake, in a large bowl, whisk the brown sugar and eggs until pale and foamy, about 1 minute. Stir in the peanut butter until smooth. Add the buttermilk, oil, vanilla and salt. Whisk until smooth and fully emulsified. Add the flour, baking powder and baking soda and whisk until well-combined and smooth.

Pour the batter into the prepared pan, tap it gently on the counter to loosen any air bubbles, and smooth the top with an offset spatula. Bake until puffed and golden, and a toothpick, inserted in the middle comes out clean, about 30 to 35 minutes. Remove from the oven and set on a wire rack to cool for 15 minutes. Then invert and flip right side up onto a plate to cool completely.

To make the icing, melt the butter, brown sugar, cream and water together in a saucepan set over medium heat. Bring the mixture to a full rolling boil and cook for 3 more minutes. Turn off the heat and let the mixture cool for 3 minutes, stirring once or twice to cool it down slightly. Whisk in the kosher salt and confectioners' sugar until smooth. Immediately pour and spread the icing over the cooled cake and sprinkle with flaky salt, if using. Allow the icing to set for about 20 minutes before slicing and serving. Store the cake, covered at room temperature for up to 3 days. Enjoy!