Preheat oven to 375, with racks in upper and lower thirds.

In a large bowl, beat together butter and sugar until pale and fluffy. Stir in the egg yolk or egg replacer mixture.

In another bowl, whisk together flour, cocoa powder, baking powder, baking soda and salt. Stir flour mixture into butter until a dough forms.

Drop dough by level teaspoons and roll into balls and place them 1" apart onto 2 parchment lined baking sheets.

Bake until cookies are puffed and cracked, about 8 minutes, rotating sheets halfway through. Remove sheets from oven and tap firmly on a counter to flatten cookies. Transfer cookies to wire rack and let cool completely.

Make the frosting: Beat caramel and butter in a small bowl until smooth; gradually beat in confectioners sugar until smooth.

Spread frosting on flat side of half the cookies and top with the remaining cookies.