Cream 10 ½ tablespoons unsalted butter and ¼ cup granulated sugar until smooth. Add 2 ½ tablespoons runny honey, ½ teaspoon lemon zest and ½ teaspoon orange zest. Mix briefly.
Add 1 large egg and mix until incorporated. Add 1 teaspoon vanilla extract and mix.
Sift 2 cups all-purpose flour, ⅓ cup cornstarch, 1 teaspoon baking powder, 1 teaspoon baking soda, a pinch of salt, and 3 tablespoons unsweetened cocoa powder. Add to the butter mixture, using a spatula to combine until a soft dough forms. Do not knead, simply integrate the ingredients well until you have an even-colored dough.
Cover the dough with plastic wrap and refrigerate 1 to 2 hours and up to 4 days.
Preheat oven to 325°F (170°C).
On a lightly floured surface, roll the cold dough about ⅙ inch. Cut plain rounds of about 2.5 inches and place on lined baking sheets. They do not expand a lot, but leave ½ an inch or so between them.
Bake 8 to 10 minutes, until set and dry but still pale. It should not be hard around the edges or colored.
Cool completely on a wire rack. At this point, you can store them in an airtight container or jar for a day or two before filling.
Place half of the cookies on a flat surface (I use a clean baking sheet), flat side up, and pipe or spoon 2 ½ cups thick or pastry dulce de leche, using about 2-3 tablespoons for each. Top with another cookie, flat side down, pressing gently to make the filling reach the edges.
Smooth the edges by running a smooth-bladed knife around them, using more dulce de leche if necessary. It will make an even wall of dulce de leche all around it.