In a large bowl, sift and mix together flour, baking powder, salt, and cocoa powder.
In a separate bowl of an electric mixer fixed with the paddle attachment, cream butter and sugar together until pale and fluffy (around 2 mins).
Add egg and vanilla extract to butter mixture and mix well until combined, using a spatula to scrape any ingredients from the side of the bowl.
Add cocoa-flour mixture to butter and egg mixture in two additions and mix until just combined and cookie dough has formed. Separate cookie dough into 2 equal sized discs. Wrap cookie dough in cling film or plastic wrap and chill for at least 1 hour up to overnight.
Once the dough is chilled, preheat the oven to 170°C (340F) and line a large baking sheet with parchment paper. Allow the dough to rest at room temperature for 10 minutes if it's too hard to work.
Sprinkle some cocoa powder or flour on your work surface and roll out the dough approx. ⅜" thick (use a rolling pin with rings to ensure even thickness). Use a Linzer cookie cutter to cut out solid round bases and use a the smaller middle shape cutter to cut out the rest of the bases (depending on size, you should have around 20 solid bases and 20 bases with shaped cut-outs).
Transfer cookies to baking sheet and bake in a preheated oven for 10-12 minutes. Once ready, allow cookies to slightly cool down and transfer to a wire rack to cool completely. Before filling with marmalade, cookies must be completely cooled.
Place orange marmalade in a microwave-safe bowl and warm for 30-40 seconds on high power.
Place around 1 tsp. of marmalade onto the bottom part of the cookie, then carefully place the top half of the cookie on the marmalade, pressing gently and being careful not to spill out any filling. Repeat for the rest of the cookies, then refrigerate for around 30 minutes until marmalade is set.
Before serving, dust with powdered sugar as desired.