Preheat the oven to 175°C (347°F).
Melt the butter using a double boiler or microwave.
Add sugar and melted butter to a bowl, and mix well.
Add the egg and vanilla extract, and mix.
Sift cake flour, salt, and baking powder into the mixture and knead into a dough.
Spread the dough evenly in a square pan.
Bake in the oven for 20–25 minutes.
Let it cool completely, then cut into desired pieces.