Place the dark chocolate in a medium heat safe bowl.
Place 1 cup of heavy cream in a medium saucepan, bring to a simmer (do not boil) whisk in the instant espresso powder until fully dissolved.
Pour the espresso cream over the chocolate chips, add the salt and vanilla extract. Let sit for 5 minutes.
After 5 minutes, whisk until the chocolate is melted and the mixture is homogenous, set aside to cool. If the chocolate hasn’t completely melted, microwave it in 20 second intervals, stirring well after each interval, until smooth. Be careful not to overheat it.
Using beaters or the whisk attachment, beat the other 1 cup of heavy cream with ½ cup of powdered sugar until stiff peaks form, set aside.
Place the mascarpone into the cooled down espresso chocolate mixture, using beaters, beat until fully combined.
Fold the whipped cream into the chocolate mixture until fully combined but do not over mix, do not deflate the whipped cream.
Fill your tart shells and refrigerate until set and thoroughly cold, about 4 hours, depending on how big your tart shells are. Enjoy the tarts cold right out of the refrigerator for the best texture.