Preheat the oven to 350° F.
Line a 9x9 pan with parchment paper and set aside.
Cream together the butter and granulated sugar until combined.
Mix in the eggs and vanilla extract until distributed evenly.
Mix in the flour, baking powder and cinnamon until a soft dough forms.
Press into the bottom of your prepared 9x9 pan.
Use the back of a spoon to 'spread' the dough smooth, giving the top a rough texture.
Sprinkle the cinnamon and sugar on top of the bars evenly.
Bake at 350° F for 25-28 minutes or until the cookie bars have puffed up and the top is beginning to turn golden brown.
Remove from the oven and allow the cookie bars to cool completely.
Cut into bars and then drizzle with the icing.
To make the icing, add the butter, powdered sugar and vanilla to a bowl and mix with an electric hand mixer, slowly adding 1 tsp of milk at a time until the icing is thick and creamy.
Microwave the icing for 5-8 seconds to warm and thin the icing just slightly.
Transfer to a piping bag and then drizzle the top of the bars with the icing. Alternately you can drizzle the warm icing on top with a spoon.
Store at room temperature for up to 3 days, in the fridge for up to 1 week or in the freezer for up to 3 months.