In a small bowl, whisk together flour, salt and baking powder and set aside.
Using a stand or hand mixer, cream together butter and sugars until light and fluffy, about 2-3 minutes. Add in the eggs one at a time, mixing well after each addition and making sure to stop and scrape the sides of the bowl with a spatula.
Add in the vanilla extract and cocoa powder, mix well, stopping to scrape down the sides when needed. Slowly add in the flour mixture, and mix until smooth on low speed to avoid over mixing.
Shape the dough into two - three discs, wrap in plastic and chill for at least two hours or overnight.
When ready to bake preheat oven to 325F.
Line baking sheets with parchment paper and set aside.
Remove the dough from the refrigerator (one disc at a time), sprinkle cocoa powder onto a counter and roll out one of the discs to 1/8 inch thick. If the disc is too hard, let it at room temperature for 10 minutes. Dust the Linzer cutter with cocoa powder and start cutting out the cookies, making sure you have equal amount bottom and top cookies. Using the center Linzer cutter, make center cut outs into the top cookies.
Transfer cookies onto the prepared cookie sheets and place at least 1-inch apart, sprinkle with sea salt if desired. Bake for about 9 minutes until the edges are firm and the centers are just slightly soft and puffed.
Remove from oven, let the cookies cool on the tray for 3-4 minutes and transfer to a wire rack to cool completely. Cookies must be completely cooled before assembling.
Place about 1/2 tablespoon of raspberry jam, or blackberry jam, or Dulce de Leche in the center of one of the bottom cookies and place one of the top halves gently on top, gently press so the jam doesn’t spill out the sides of the cookie. Repeat with remaining cookies.
Optional: before assembling the top cookies can be dusted with powdered sugar for the raspberry and blackberry version.
Store in an airtight container at room temperature for one week or frozen for up to 3 months.