Pre-heat oven to 350F / 180C. Line a 7 inch square baking tin with parchment paper and grease sides well with butter.
Combine flour, salt, baking powder, baking soda, cardamom powder in a bowl. Set it aside.
Whisk the butter and sugar until fluffy and pale in color – about 10 minutes by the manual whisk, 3-4 mins with a hand mixer at medium-high speed.
Add egg one at a time, beating each time for about 10 - 20 seconds using a hand mixer at medium speed. In between add vanilla essence as well.
Add the flour mixture, and fold until all the ingredients are combined nicely. Add the food color and gently fold it into the batter.
Pour the cake batter into the greased pan. Bake in a preheated oven for 30 - 35 minutes, or until a toothpick comes out clean. Let the cake sit in the tin for 10 - 15 minutes.
Turn the cake onto a cooling rack and let it cool for 15 - 20 minutes. Then, using a sharp knife, cut it down the middle, then into ¼ inch thick strips.
Divide each strip into 2 - 3 rectangular pieces (rusk) depending upon the size of the strip.
Arrange the rusks on a baking sheet, and put the sheet back into the oven.
Bake at 150 degrees Celcius for 15 minutes. Turn them over and bake the other side for another 15 minutes. If they need more time in the oven, turn down the heat a little bit and let them bake for slightly longer until they are golden brown and crisp on all sides.
Transfer warm cake rusk onto a wire-rack and let them cool. Store in an airtight container. Serve cake rusk with a cup of tea or coffee.