In a large mixing bowl, cream together softened unsalted butter and granulated sugar until light and fluffy, about 2-3 minutes. Gradually add in all-purpose flour, cocoa powder, vanilla sugar, and the large egg, mixing until a smooth dough forms.

Wrap the dough in plastic wrap and refrigerate for 30-60 minutes; the dough should be firm but pliable when ready.

While the dough chills, prepare the whipped white chocolate cream. Start by bringing heavy cream to a gentle boil in a small saucepan. Pour the hot cream over finely chopped white chocolate in another bowl, stirring continuously until completely smooth and glossy.

Chill the mixture in the refrigerator for about 1-2 hours to firm up before whipping.

Preheat your oven to 160°C (fan) or 180°C (conventional) and line a baking sheet with parchment paper.

On a lightly floured surface, roll out the chilled cookie dough to a thickness of about 3-4 mm. Use a round cookie cutter to cut out circles, and then cut out the centers from half of them to create windows.

Place the cookies on the prepared baking sheet and bake for 10-12 minutes; they should feel firm yet slightly soft in the center.

Once the cookies have cooled completely on a wire rack, melt dark chocolate in a microwave or double boiler. Dip the tops of the window cookies in the melted dark chocolate and sprinkle with finely chopped dark chocolate.

Set them aside on parchment paper until the chocolate hardens.

Retrieve the chilled white chocolate cream and whip it with an electric mixer until fluffy and airy, about 3-4 minutes.

Pipe a ring of the whipped cream onto the base cookies, then fill the centers with warmed sieved cherry jam mixed with optional cherry liqueur. Finally, place the decorated window cookies on top, gently pressing down to complete your gourmet Black Forest cherry cookies.