To make the cupcakes:

1. Preheat the oven to 350 degrees. Line a 12-cup muffin tin with liners. Set aside.
2. In a medium mixing bowl, whisk together the flour, cocoa powder, espresso powder (if using), baking powder and salt. Set aside.
3. In the bowl of a stand mixer fitted with the paddle attachment (or in a large glass mixing bowl with a hand mixer), beat the oil and sugar together until fluffy. Add one egg at a time and beat until light in color. Beat in the vanilla extract.
4. While beating on low speed, alternate adding ⅓ of the dry ingredient mixture and ⅓ of the milk into the bowl with the oil/sugar mixture, ending with milk, mixing just until incorporated.
5. Use a measuring cup or cookie scoop and fill each cupcake liner about ¾ of the way full. Bake 15-20 minutes or until a toothpick inserted into the center comes out clean. Let slightly cool, then remove from each cupcake from the muffin tin and set on a wire rack to completely cool.

To make the frosting:

1. Place the chocolate chips in a microwave safe bowl and microwave on high in 30 second increments, stirring the chocolate in between each, until it’s melted.
2. In the bowl of a stand mixer fitted with the paddle attachment, whip together the softened butter and cream cheese until fluffy. Slowly pour in the melted chocolate. Add ¼ cup of the powdered sugar at a time and whip until fluffy. Add the vanilla extract and a pinch of salt (if using).
3. Pipe or slather frosting on top of each cupcake and top each with crushed walnuts (if using).