Preheat the oven to 350 degrees F. Line 2 baking sheets with parchment paper. Set aside.
In the bowl of an electric mixer fitted with the paddle attachment, mix together the butter and sugars, until light and fluffy.
Add in the egg and vanilla, and mix.
With the mixer running on low, add the flour, cornstarch, baking soda, salt, pudding mix, cocoa, and coffee. Once the dough comes together stop the mixer. The dough should be thick but not sticky at all.
Form tablespoon size balls, and push the tops down slightly. You want them to be thick and tall, so don’t flatten too much. This is what makes the cookies thick and rounded after they bake.
Place them on the baking sheets and bake for 7 minutes. Allow them to cool for 5 minutes on the baking sheet, and then transfer them to a cooling rack to cool completely.
For the Buttercream
In the bowl of an electric mixer, cream the butter for 2-3 minutes until light and fluffy. Add in the vanilla and ½ cup of the powdered sugar and mix until incorporated. Keep adding ½ cup of the sugar at a time, scraping down the bowl as necessary. Once all the sugar is added in, mix on high for about 2-3 minutes to aerate the buttercream. You should be left with fluffy and creamy buttercream.
I used a pastry bag with a giant star tip to fill the cookies, but you can use a ziplock bag or even a spatula (for a more homemade look). The trick is to add a nice tall layer of buttercream so the sandwiches are thick and have height.
Recipe by My Fair Hostess at https://rachelkor.com/chewy-chocolate-sandwich-cookies-with-vanilla-buttercream/