Preheat oven to 350°F.

In a small saucepan, cream together the butter and sugar.  When smooth, add the molasses and gingerbread spice mix.  Mix together, then bring to a boil stirring constantly.

Remove from heat and allow it to completely cool.

When cool, mix in the eggs and vinegar.  Lastly, add the flour and baking soda to the molasses mixture and stir until a soft dough forms.

Line a cookie sheet with parchment paper, then pat out the dough into a thin layer.  Chill for at least 20 minutes.

Cut out cookies and bake on prepared cookie sheet (I use parchment paper or silicone baking mats).  Bake for 8-10 minutes.  Move the cookies to a cooling rack.

While the cookies are cooling - make the frosting.

In a small saucepan, whisk together the flour and eggnog, stirring contstantly.  It will get very thick, very quickly.  As soon as it's thick like batter, remove from heat and allow it to cool.  This mixture will be sort of chunky - that's okay!  It must be completely cool before you use it.

As it cools, in a mixing bowl, cream together the butter and sugar until it's light and fluffy and no longer grainy.  Then add the cooled eggnog/flour mixture and the vanilla.

Continue mixing for several minutes until it's very light - like whipped cream.  This may take awhile - if it looks separated, keep mixing!

Assemble the cookies by flipping one upside down, piping or spooning on a dollap of buttercream, then top with another cookie.  Chill for at least an hour - enjoy!

This recipe makes about 20 sandwich cookies total.