Preheat oven to 350°F (175°C). Grease and line a 9-inch cake pan with parchment paper.
In a bowl, whisk flour, baking powder, baking soda, and salt—set aside.
Cream together butter and brown sugar until fluffy.
Beat in eggs, one at a time, then add vanilla extract.
Alternate adding flour mixture, espresso, and buttermilk, mixing gently.
Pour batter into the cake pan, smoothing the top.
Bake for 35 minutes or until a toothpick comes out clean.
While baking, make the caramel sauce: Melt sugar in a saucepan until golden.
Slowly add cream, butter, espresso, and salt while whisking. Remove from heat.
Let the cake cool completely, then drizzle with caramel sauce.
Slice, serve, and let the sugar rush begin.