Beat the butter until light and fluffy. Add both sugars and beat to combine. Add the sifted cocoa powder, mix to combine.
Add the vanilla and egg, mix well.
Mix in the flour and salt. Mix until the dough comes together.
Shape the dough into two large discs, wrap in plastic wrap and chill for 1 hour.
Preheat the oven to 350F/175C
On a floured surface roll the dough out to about 1/4 inch thick.
Using a 2.5-inch cookie cutter, cut out the cookies and then using a 1-inch cookie cutter, cut a hole in the center of 1/2 of the cookies.
Place the cookies on a cookie sheet (about an inch apart) and bake 10 -12 minutes until set (being careful not to overcook the edges. Let cool completely before assembling.
To assemble, spoon a heaping teaspoon in the center of the cookie without the hole, spread slightly, then using a cookie with a hole, press the cookies together gently. The fluff will spread as it sets.
Let the cookies set for 2 hours before serving. Store in an airtight container, the cookies will keep for 5 days.
How to Make Perfect Sandwich Cookies
Rolling the Cookies
There's a good amount of butter in this recipe, which is a good thing for taste and texture but could make the dough a bit more difficult to roll out. Chilling the dough beforehand makes the rolling easier.Most recipes typically call for rolling the dough out while it is soft, but giving this dough an hour in the fridge makes it less sticky and easier to roll. Dividing the dough into two sections also makes rolling easier.
Making Sandwich Cookies The Same Size
Having two cookies the same size is the key to a perfect sandwich cookie.Once you've rolled and cut, I recommend chilling the cookie dough again before baking to ensure the cookies retain their shape and size.
Cutting the Top Cookie
To get the cutout that displays the filling, you can use any shape of nesting cutters you want. Don't be afraid to mix shapes to create unique shapes. I used hearts for Valentine's Day, but the same cookies can be made in many different shapes. If you want to make hearts like mine. Click here to get a similar set on Amazon.
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Top tip: Try to bake the tops with the center cut out on their own cookie sheet, as they tend to bake just a little faster.
Assembling the Cookies
Don't add too much, my filling. It will ooze out the sides and make a mess.Add a heaped teaspoon in the middle, spread it just a little, and then press the cookies together. As the cookies set, the fluff spread on its own to fill the inside.