For the Cupcakes:

1. Preheat the oven to 350°F (175°C). Line a muffin tin with cupcake liners.
2. In a medium bowl, sift together the flour, cocoa powder, baking soda, baking powder, and salt.
3. In a separate bowl, cream the softened butter and granulated sugar together until light and fluffy. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
4. In a small bowl, combine the cooled espresso and buttermilk.
5. Gradually add the dry ingredients and the espresso mixture to the butter mixture, alternating between the two, starting and ending with the dry ingredients.
6. Divide the batter evenly among the cupcake liners, filling each about 2/3 full.
7. Bake for 18-20 minutes, or until a toothpick inserted into the center of a cupcake comes out clean. Let the cupcakes cool completely in the tin before transferring to a wire rack.

For the Salted Caramel Buttercream:

1. In a large bowl, cream the butter until smooth.
2. Gradually add the powdered sugar, beating well after each addition until the mixture is light and fluffy.
3. Stir in the salted caramel sauce and mix until smooth and creamy.
4. Frost the cooled cupcakes with the salted caramel buttercream.