Preheat oven to 180 c and prepare an 8 x 10 in baking pan by lining it with parchment paper.
Sift the flour with baking powder and salt in a bowl. Set aside.
In a mixing bowl, using a hand mixer, beat the butter and sugar until fluffy and pale in color.
Add the yellow food color (if using) and mix well. Add the eggs one at a time, beating well after each addition.
Now add in the flour and mix until just combined. Lastly, add the vanilla extract and fold it in gently.
Pour the cake rusk batter into the prepared baking pan and bake for about 15-20 minutes or until a toothpick inserted comes out clean.
Remove the cake from oven and let cool. Then remove the cake from the baking pan, and let it completely cool. Reduce the oven temperature to 150 c.
Once the cake has cooled, use a sharp knife to cut the cake into 1 inch strips.
Transfer the cake slices to a baking sheet and bake for 15 minutes. Remove the baking tray from the oven, turn the cake rusk sides and bake for another 15 minutes, until golden brown and crisp.
Remove from the oven and cool on a wire rack.