You can make this versatile mix into any-shaped shortbread. I doubled the recipe to fit into a 33cm x 23cm slice tin, as I wanted rectangular pieces to fit into our daughter's lunchbox. One batch will fit a 20cm round pan to make 12 triangle pieces.

1/3 c granulated sugar
1/4 tsp sea salt
zest of a lemon
115 g butter at room temperature
1 tsp vanilla extract
1/4 tsp lemon oil or extract (I used some lemon juice)
1 cup plus 2 tbsp (150g) flour
1 tbsp poppy seeds, plus extra for sprinkling

Lemon drizzle: mix 1/2 c icing sugar with 2 tbsp lemon juice

Preheat oven to 170C. Line a 20cm round pan with baking paper and dust with flour.
In a large bowl rub the sugar, salt and zest with your fingers until sugar is moist and fragrant. Add the butter and blend with a hand mixer or stand mixer paddle until smooth, about 2 minutes. Beat in vanilla and lemon extract. Add all the flour at once and mix on low speed until just combined, then mix in poppy seeds. You should have large moist crumbs that hold together when pressed.
Turn dough out onto your tin or pan and spread evenly, using the bottom of a cup to smooth out. Use a knife to score your shortbread shapes: triangles if using a round tin or rectangles/squares/diamonds if using a slice tin. Prick each piece once or twice with the tines of a fork. In a round tin you can also crimp the edges with a fork.
Bake for around 25 min until edges are golden brown but the centre should remain fairly pale. Rotate tray halfway through baking.
Remove from oven and let cool in the tin for 5 minutes over a wire rack. Then turn out pieces onto your rack and leave to cool completely. Drizzle over lemon icing once cooled and sprinkle with poppy seeds if desired.