Preheat the oven to 175 C. Line 6 muffin moulds with liners. If you prefer smaller cupcakes, use 8 muffin moulds.
Sift the flour, baking powder and cinnamon in a large bowl. Mix in the salt if using, then set aside.
Beat the butter and sugar in a mixing bowl till pale and fluffy.
Add the cookie butter and beat lightly. Add the eggs and vanilla and beat well till combined.
Fold in the flour mixture, and beat gently until smooth and combined, adding the milk gradually for a thick but spreadable batter. You may not need all of the milk.
Fold in the crushed biscuits and divide the batter equally among the prepared moulds.
Bake for 20 mins, then another 5 to 10 minutes till lightly browned and a toothpick poked in the center comes out clean. Let the cupcakes cool completely at room temperature. They will be on the taller side, not flat, because this is technically a cake batter, not specifically a cupcake batter.
To make the frosting, beat all the ingredients together till smooth and combined. You can thicken the frosting with more sugar and butter, but I found this just the right amount of sweet. You can chill it for 15 minutes to firm it up, then frost the cupcakes, decorate with biscuits and dig in! Store the cupcakes in the fridge at all times, they will keep for a week or so, but leave them at room temperature for about 20 minutes before eating. Happy baking!