Preheat oven to 350 degrees. Line 2 baking sheets with parchment paper or silicone baking mats.
Combine the chocolate and butter and microwave for 1 – 2 minutes, stirring every 30 seconds, until most of the chocolate chips are melted. Stir until smooth. Let cool slightly.
In the bowl of an electric mixer, combine the eggs, sugar, and 1 teaspoon vanilla and beat on high for 5 minutes or until very light and fluffy. Combine the flour with the baking powder and salt, and add to the eggs. Beat until combined. Add the melted chocolate and beat until combined. Scrape the sides and bottom of the bowl with a rubber spatula to ensure everything is well mixed.
Drop tablespoonfuls of the batter onto the prepared baking sheets (a small ice cream scoop works well for this). Bake for 10 – 12 minutes or until the cookies are set and start to crack. Let cool completely.
In a clean mixing bowl with clean beaters, combine all the filling ingredients and beat until smooth.
Spread the filling on half of the cookies on the flat side. Top with the remaining cookies.