In a large bowl or the bowl of a stand mixer, add the butter and sugar. Mix well until butter is fluffy. Add in the eggs, one at a time, making sure it is fully incorporated before adding the next.
Add baking powder, green cardamom powder, and flour and mix just until incorporated, don't over mix.
Prepare an 8x8 inch square baking pan lined with parchment paper.
Add the mixture and tap the pan on the countertop to release any air bubbles.
Bake at 350ºF for 25-30 minutes or until the toothpick inserted comes out clean.
Cool the cake completely.
Once cooled, slice the cake into rectangles. Lay the slices flat on a baking tray. Bake at 300ºF for 30 minutes. Turn them over, and continue cooking for 15-20 minutes until the cake rusks are dry and crunchy.
Cool the cake rusk completely.
Transfer to an airtight container and store for 1-2 weeks.