Beat the butter and cream cheese until light and smooth.
Add the sugar and salt, mix until combined.
Add the egg, mix until combined, then add the egg yolk and mix until blended.
Add the flour in batches, mixing after each addition.
Form the dough into a flattened disc, wrap in cling film or slip into a resealable bag and chill for several hours until firm.
Preheat the oven to 325F (yes, slightly lower than the usual 350F standard baking temperature because of the cream cheese content of the dough).
Line a baking tray with parchment paper.
Form the dough into balls no more than an inch in diameter. Flatten slightly between your palms and arrange on the baking tray about two inches apart.
Press a chocolate disc onto the top of each cookie.
Bake for 15 to 18 minutes or until the cookie bottoms are lightly browned. The cookies will spread and flatten a bit during baking.
Cool the cookies on the tray for about three minutes then transfer to a wire rack to cool completely. Dust with confectioner’s sugar and serve.