This classic recipe calls for simple everyday ingredients. *Tip: use high quality chocolate products
Whip the butter and sugar until combined, then mix in the egg yolk, milk and vanilla.
Add the dry ingredients: flour, cocoa powder and salt. Mix just until combined.
Roll the dough into small balls, then use the back of a measuring spoon to dent each ball at the center. Refrigerate, then bake.
As soon as the cookies come out of the oven, use the back of the spoon again to press down the center where you dented.
Once the cookies have cooled completely, fill with chocolate ganache, allow to set, then serve.