Whip butter and sugar - Add your softened butter, sugar, salt, vanilla extract and orange zest (optional) to a medium sized mixing bowl. Use an electric hand mixer or stand mixer fitted with the paddle attachment to cream the ingredients together until pale and fluffy, about 3 minutes.
Add dry ingredients - Add half the flour and cocoa powder (sifted) and use a spatula to mix until a dough forms and no dry ingredients are showing. Then add the remaining dry ingredients and repeat. The dough will be soft and smooth when pressed together. If it’s a warm day the dough may be too soft to work with so pop it in the fridge for 30 min to firm up before moving to the next step.
Rolling out the dough - Form the dough into two flat oval shapes using your hands. Place one dough disc on top of a large piece of baking paper and place another large piece of baking paper on top. Roll out to about 1/2cm / 0.20-inch in thickness. Repeat with the other dough disc.
Chill the dough - place the rolled dough onto a large baking tray and chill in the fridge for an hour.
Preheat your oven to 150C / 300F (fan off), 130C / 265F (fan on).
Cutting out the biscuits - Use a 4cm / 1.5-inch round cookie cutter to cut out your cookies. I used fluted edged ones.
Baking - Place on the baking tray, spacing about 2cm apart to allow them to spread slightly. Bake for 20 minutes. They’ll be slightly soft when they come out of the oven but will firm up as they cool.
To assemble - Fill half of the cookies with dulce de leche and sandwich with the other half. I did this by using a piping bag and snipped the end off. You can also use two teaspoons to do this.