With a stand mixer and whisk attachment, beat the butter until smooth. WITH THE MIXER OFF!!!! (I learned this the hard way) add in the sugar and salt. Beat on low speed until the sugar and the butter has been combined. In a small cup, combine the instant espresso powder, vanilla, and cream. Pour the espresso/vanilla mixture into the mixer and beat on medium until combined. Increase speed to medium-high, and beat until light and fluffy, about four minutes. Use to frost your cake!