Use a mixer to whisk together the butter, cream cheese, and 2 tbsp of sweetener till a smooth, homogeneous mixture is obtained. Crack the eggs in the mixer and add the baking powder as well. Blend for an additional 3 minutes. Put the mix aside.
Pour the water into a small skillet together with the rest of the sweetener. Set the stove to low heat. Crush half of the berries and gently throw them into the water. Allow cooking for 10 minutes to obtain a jelly-like mixture.
Transfer the jelly mix and the cream cheese mix into a greased cupcake cup. Place the whole berries on top and use a fork to insert these into the base of the cup. Pour some water into a baking pan and arrange the cupcake cup on the pan.
Leave in the oven to bake for 35 minutes. Simply insert a knife into the cake to check if it is cooked through, and see if the knife comes out clean.