Preheat the oven to 175 C. Line a baking tray with a silicone mat or lightly grease the tray (use two trays if you have a small oven and bake in batches).<br></br>

In a mixing bowl, beat the butter and sugar till pale and fluffy. Add the egg yolk and vanilla and beat to combine.<br></br>

Add the flour and salt and fold gently to form a soft, sticky dough. Though it is quite thick, it should be of a pipe-able consistency. If it feels too thick, add a splash of milk.<br></br>

Spoon the batter into a piping bag fitted with a tip that's about 1/2 inch wide. The dough will not be easy to pipe if you use a very thin tip. I used a star shaped one here to get the rose like cookies, but you can use any one you like. Pipe cookies that are about 2 inches in diameter, spacing them an inch apart on the silicone mat. Alternately, drop small dollops of the dough onto the tray and flatten lightly, if you don't want to pipe them.<br></br>

Bake for 10 minutes, then another 2 to 3 minutes to let them become golden brown and crispy edged. Rotate the tray to ensure even browning.<br></br>

Allow the cookies to cool on the tray, then place on a wire rack to cool completely.<br></br>

Leave them plain, or dip one edge of each cookie in melted chocolate and cover immediately with sprinkles. Plain cookies will keep in an airtight tin at room temperature for 2 days but personally I feel they taste better cold from the fridge. The chocolate covered ones should be stored in the fridge from day one itself. Happy baking!