These chocolatey cookies, which are filled with a vanilla-infused ganache, have a texture similar to brownies.
Preheat oven to 350 degrees. Sift flour, cocoa powder, and salt into a small bowl. Cream butter and sugar with a mixer until pale and fluffy. Reduce speed to medium, and add yolks, cream, and vanilla. Scrape sides of bowl. Beat in flour mixture until just combined.
Roll balls using 2 teaspoons dough for each, and roll each in sugar. Place 1 inch apart on parchment-lined baking sheets. With the handle of a wooden spoon, press gently in the center of each to create an indentation. Bake, rotating sheets halfway through, until cookies are just set, about 10 minutes. (If indentations lose definition, press centers again.) Let cool slightly on baking sheets. Transfer cookies to wire racks, and let cool.
Spoon warm ganache into center of each cookie. Let stand until firm, about 15 minutes.
Cookies will keep, covered, for up to 3 days.