Preheat the oven to 180 degrees Celsius ( 350 Fahrenheit, 160 fan oven).
To make the sponge, add the soft butter and granulated sugar to a large mixing bowl, and use a hand mixer to beat them well until you have a light texture.
Add the eggs one by one, beating well after every addition.
Sift in the flour, add the bicarbonate of soda, baking powder and vanilla extract, and use a spatula to mix everything into a batter.
Spread the melted butter over the bottom of a square baking tin ( mine is 24 cm large and 5 cm high - 9 inches large and 2 inches high).
Sprinkle the dark brown sugar over the melted butter, then arrange the fig slices.
Spread the batter over the fruit, then bake for 30 minutes until the top is golden, and a skewer inserted in the middle comes out clean.
Leave the cake to cool down slightly, then run a sharp knife around the edges of the sponge to loosen it up.
Place a wooden board on top of the tin, and turn it upside down quickly.
Slice the cake into squares and enjoy.