Heat your oven to 180°C/160°C/355°F/Gas Mark 4.
Place your muffin cases in your muffin tin.
Beat together your softened butter and brown sugar until soft, fluffy and paler in colour. Start on a low setting and increase to medium-high as the mixture softens. It will take 2-3 minutes to reach the correct consistency. (See FAQs above if your brown sugar is hard).
Sift together your flour and baking powder.
Add a third of your beaten eggs and a heaped tablespoon of your sifted flour mixture to your butter mixture and beat on a low setting. Stop as soon as soon as you can no longer see any egg.
Repeat twice more until all of your egg mixture as been mixed in.
Add the rest of your flour mixture and beat until just incorporated. Scrape down your bowl using your spatula to ensure all of the mixture from the sides and bottom is mixed in.
Add your milk and then beat again for about 10 seconds
Using one teaspoon to scoop up some batter and one to scrape it off into the muffin case, add equal amounts of batter to each muffin case (see Notes below).
Place into your oven and bake for 18-20 minutes. When ready your cupcakes will be golden on top and spring back up when you gently press them.
Remove to your cooling rack and leave to cool completely.
Once cool use your cake corer or a knife to cut out little holes in the top of each cake, about 2 cm deep. Put the removed cake aside.
Fill these holes with your Biscoff spread. If your Biscoff is a little stiff pop it into a microwavable bowl and heat for 10 seconds.
Replace the removed cake pieces to hide the Biscoff inside.