Cut the butter into 6 pieces. The butter should be cold and each piece should be about 1 tablespoon. Set into a medium bowl
Chop the chocolate into small pieces and add into the bowl with the butter along with the salt and vanilla. Set aside
In a small saucepan over medium heat, heat cream and sugar until hot but just before boiling
Once the cream is heated, mix in the espresso powder
Pour the heated cream mixture over the chocolate, butter, salt, and vanilla. Let sit for 1 minute then mix until incorporated
Set in the fridge, stirring every 15-20 minutes until cooled but not hard. The icing should be a spreadable consistency
Espresso Cream:
In a large bowl, combine the mascarpone cheese and heavy cream. Beat on medium speed until well combined, about 30 seconds.
Add the sugar, salt, vanilla bean paste, espresso powder, and espresso grounds. Continue to beat on medium-high speed until soft peaks form, about 1-2 minutes. Do not over mix or the mixture will split. You want to just mix until soft peaks form
Assembly:
Prepare the Layers: Level the tops of the cooled cakes with a serrated knife to create even layers.
Place the first cake layer on a serving plate or cake stand. Pipe a border around the edge so there is about ¼ inch thick border of chocolate ganache icing around the whole cake layer (this will keep in the mascarpone). This can be done with a ziploc filled with ganache that one of the corners has been snipped from
Top with a generous layer of the mascarpone cream and repeat one more time. Top with the third cake layer.
Coat the entire cake in the remaining ganache icing