Preheat your oven to 350°F (175°C) and line a small baking dish (around 8x8 inches) with parchment paper.
In a mixing bowl, beat the butter and sugar together until smooth and creamy. It should look pale and fluffy. This step gives you that signature melt-in-the-mouth feel.
Add vanilla extract if using, then slowly add the flour and salt. Mix until the dough just comes together. Don’t overmix. It should be soft but not sticky.
Press the dough evenly into your prepared pan using your fingers or the back of a spoon. Smooth the surface gently.
Bake for 30–35 minutes until the top is lightly golden and the edges are set.
Let it cool for 10 minutes in the pan, then cut into biscuit bars while still warm. Leave them to cool completely before serving so they firm up nicely.