Cream butter and powdered sugar.
Add yolks, espresso, vanilla, flour, and salt.
Chill dough 30 min.
Shape into 1-inch balls and flatten.
Bake at 350°F for 12–15 min.
Whip cream, beat mascarpone.
Add sugar, vanilla, espresso to mascarpone.
Fold in whipped cream.
Pipe filling on half of cookies.
Brush cookies with espresso.
Sandwich and dust with cocoa.
Chill for 2 hours before serving.