Preheat oven to 180C/350F and grease and line an 8x8 inch square pan.

Place butter and sugar into a large bowl, or the bowl of your stand mixer, and beat until light and fluffy. Around 2-3 minutes on medium-high speed.

Add in egg and vanilla extract, scraping down sides of bowl as necessary and mix until well incorporated and smooth.

Add in the flour, baking powder, bicarb and salt and mix on low until the dough just comes together. It won't be light and fluffy like a normal cookie dough, but if you pick up a chunk of dough and press it between your fingers, the dough will hold together.

Tip the dough into your prepared pan and press into a level layer using a spatula/wooden spoon or your hands. I used my hands as I found it easiest!

Place in the oven for 15-20 minutes, until puffy and very lightly golden all over. Cookie bars will also feel slightly firm to the touch.

Leave to cool in the pan completely, before transferring to a cutting board, then make the frosting.

Place butter, icing sugar and salt into a large bowl, or the bowl of your stand mixer and beat on low at first until sugar begins to incorporate. Then turn the mixer to high and beat until light and smooth.

Add in vanilla and milk, if required, and mix until smooth and light. You want it to hold its shape, but be soft enough to spread.

Spoon the frosting into the centre of the slab of cookie and use a small angled spatula to spread and smooth all the frosting over the cookie bars.

Sprinkle whichever sprinkles you want all over the frosting and then slice into 9-12 bars. I find it easiest to use a pizza cutter to cut the cookies into bars.

Frosted Sugar Cookie Bars will keep in an airtight container, in the fridge for 5 days, or at room temperature for 3 days.