1. To make the chocolate wafers, whisk the flour, cocoa powder, baking powder, and salt together. In another bowl cream the butter and sugar until light and fluffy. Add the egg yolk and vanilla. Mix until incorporated. Add the flour mixture to the butter and combine until the dough comes together. Divide the dough into two discs, wrap with plastic, and chill for 2-3 hours, overnight, or up to three days.

2. Make the ganache by heating cream and corn syrup together. Once it comes to a simmer, take off heat. Stir the cream into the melted white chocolate. Add the butter and vanilla. Chill for 30 minutes.

3. Preheat oven to 350 degrees F or 170 degrees C.

4. Bring cookie dough out of fridge 30 minutes before rolling. Once workable, roll to 0.3 or 0.4 cm thick. If the dough becomes too soft, move to tray and place in the freezer for a few minutes. Cut out as many rounds as possible (I used a 6 cm fluted round, Gourmet used a 4.4 cm one) and place on a parchment lined baking sheet. Combine scraps and form into a ball. Chill in the fridge for 10 minutes and re-roll one more time. Repeat with second disc. When the rounds are all ready, chill in the fridge for a few minutes. Right before baking sprinkle with decorative sugar. Bake for 10-12 minutes, rotating the baking sheets around in the middle. Cool completely.

5. Once the cookies are completely cooled, prepare ganache by beating with an electric beater until light and fluffy. Place into a piping bag and pipe onto half the cookies. Sandwich together with the remaining rounds.