• Preheat oven to 350 degrees (F). Line a cupcake pan with liners; set aside.
• Place the butter and chocolate in a heat-proof bowl and place that bowl over a saucepan of gently simmering water, making sure the bottom of the bowl does not touch the simmering water. Stir until the butter and chocolate are completely melted. Add in the brown sugar, espresso powder (if using), and vanilla; whisk well to combine. Remove from heat and allow mixture to cool for a few minutes.
• Once the mixture has slightly cooled, add in the eggs and yolk, one at a time, beating well after each addition. Whisk in the flour, salt, and baking powder - careful not to over mix here! Fold in the chocolate chips.
• Divide the batter among the prepared cupcake cups.
• Bake until a wooden pick inserted in the center comes out clean, about 21-25 minutes. Allow the cupcakes to cool in the pan for 20 minutes before transferring them to a wire rack to cool completely.
• In the meantime, make your frosting!

For the Salted Caramel Frosting:

• In a medium-sized saucepan over medium-heat, combine the butter, cream, vanilla, brown sugar, and salt. Cook, stirring occasionally, until the sugar melts and the mixture comes to a low simmer; about 4 minutes. Remove the mixture from heat and let it cool for 2 minutes, then transfer the mixture into the bowl of an electric mixer fitted with the whisk attachment. Add 2 cups of confectioners' sugar to the bowl and beat on medium-low speed until well combined. Add in another 1-2 cups of confectioners' sugar and beat until thick and fluffy; 4-6 minutes. Frost cooled cupcakes as desired.