In a mixing bowl, whisk eggs and vanilla. Followed by butter and sugar. Lastly, mix in flour and baking powder.
Grease the bottom of a 9-inch square baking tray with more butter and lay it on parchment paper.
Pour the cake rusk batter into the tray and evenly spread it around with a spatula.
Bake in the oven for roughly 30 minutes.
Remove the cake onto a cutting board. Reduce oven temperature to 350 °F (175 °C). Slice the cake into rectangles.
Place rectangles on a larger baking sheet tray and place it back in the oven for an additional 10 to 15 minutes. Flip rectangles and back in the oven for the same amount of time so each side gets a bit golden brown.
Enjoy immediately with chai or store in an air-tight container.