• Preheat oven to 175°C (350°F) and line two baking trays with baking paper.
• Whisk the flour, cocoa powder, baking powder, and salt together in medium bowl. Set aside.
• In a large bowl using a handheld mixer, or in the bowl of a stand mixer fitted with a paddle attachment, beat the butter and sugars together on low-medium speed until smooth and creamy, about 30 seconds. Add the vanilla extract and beat, scraping down the sides as needed, until the batter is combined - around a minute or so.  Pour the dry ingredients into the wet ingredients, add the milk, and mix on low until combined. The cookie dough will be thick and tacky but not sticky.

  Note: Sticky dough is when the dough sticks to your hand, leaving a mess. Tacky dough sticks but then releases from your hand, leaving no mess.
• Roll balls of dough roughly a heaped half tablespoon in size (40g). If you like, you can pop 2 tablespoons of granulated sugar in a bowl and roll ball each through to coat. Place dough balls on the prepared trays about 2-inches apart.
• Using your thumb or a measuring teaspoon make an indent into each cookie. If you find that the balls are cracking when you make an indent, use your hands to push back together. (Some cracks are bound to happen as the cookies expand in the oven– see my photos. It's all good!)
• Bake for 11 minutes or until the edges are set. The cookies will still look and feel a little soft. Remove from the oven. The indents will have lost some shape or have puffed up, so use your measuring spoon to make an indent again into the warm cookies. Don't use your thumb! The cookies will be hot.
• If you are filling your cookies with Biscoff, carefully fill each cookie with around a half teaspoon of the spread while the cookies are still hot. The Biscoff will melt but set again as the cookies cool.
• If you filling your cookies with chocolate ganache, allow the cookies to cool completely.

For the ganache.

• Place the chocolate into a heat-proof bowl. Heat the cream in a small saucepan until it begins to gently simmer. (you are looking for small bubbles around the outside of the pot, don't let it it boil) Pour the hot cream over the chocolate and let it sit for 2-3 minutes to gently soften the chocolate. Slowly stir until completely combined and chocolate has melted. Add a small pinch of salt and stir it through.
• Fill each cookie with a half to full teaspoon of the ganache (depending on the size of the indent) and set aside to set for around 30 minutes.
• And you're done. The cookies stay fresh covered at room temperature for up to 3 days or in the refrigerator for up to 1 week.