Preheat oven to 375 degrees F. Line a baking sheet with parchment paper.
Add 1 cup water to a pot. Bring to a boil and then lower the temp to a simmer.
Put the unsweetened and semi-sweet chocolates and butter in a medium metal mixing bowl.
Using the double boiler method, melt the butter and chocolate chips together. When done, remove from the stove.
Whisk the sugar, eggs and vanilla into the chocolate mixture until smooth.
Sift the flour, cocoa, baking powder and salt into another bowl.
Gradually whisk the dry ingredients into the wet ingredients until moistened.
Switch to a spatula and finish folding the batter together; take care not to over-mix.
Use a small cookie scoop or spoon to drop a heaping tablespoon of batter onto the prepared baking sheet. Repeat until the batter is finished, spacing them about 1-inch apart.
Bake until the cookies spring back when lightly touched, about 6 minutes.
Cool the cookies slightly on the baking sheet and then transfer them onto a cooling rack.
Once cooled, fill one side of the cookie with the swiss meringue buttercream and top with another cookie.
Continue until all the cookies are done.
Store in tightly sealed container for up to 1 week.