Preheat the oven to 350°f / 180°c. Grease and line a 9x13" (20x30cm) pan with parchment paper, leaving a little overhang or piece sticking out so that you can remove the cooled cookie bars once done.

In the bowl of a stand mixer fitted with the paddle attachment or using a handheld electric mixer, cream the butter and sugar together on high speed until light and fluffy. Add the egg and vanilla and mix to combine. Sift together the flour, salt, and baking soda in a small bowl. Add to the mixer, and mix until just combined.

Transfer the cookie dough to the prepared pan, and spread out into an even layer using an offset spatula.

Sprinkle the top of the cookie bars with granulated sugar if desired.

Bake the sugar cookie bars for 20 to 25 minutes, until they are lightly golden brown on the tops and the center looks set. They will crinkle a little around the edges of the pan and the cookie bars will deflate slightly when removed from the oven.

Remove the cookie bars from the oven and place the pan on a wire rack to cool completely.

In the bowl of a stand mixer fitted with the paddle attachment, whip the butter, vanilla bean paste and salt until pale and creamy. Sift in the powdered sugar, and mix on medium speed until well combined.

If needed, add cream a tablespoon at a time, until the frosting is light and fluffy, and a spreadable consistency. Add gel food colouring until the desired colour is reached.

Run the mixer on low speed for 2-3 minutes to help remove any air bubbles from the frosting. You will need to use the frosting immediately or place a piece of plastic wrap directly against the surface to stop it from crusting.

Remove the cooled cookie bars from the pan using the parchment paper and place onto a cutting board or serving platter.

Transfer the frosting to the cookie bars and swoop it on using the back of a spoon or an offset spatula. Add sprinkles if you like.

Chill the cookie bars just until the frosting is set to help cut them into clean pieces. Cut into pieces with a sharp knife, wiping the knife between cuts.

Store cookie bars in an airtight container in the fridge or at room temperature for up to five days.