Preheat the oven to 180 degrees Celsius (350 Fahrenheit, 160 degrees fan oven).
To make the sponge, add the light brown sugar to a bowl together with the butter, and use a hand mixer to beat them together until creamy.
Add the eggs one by one, beating well after each addition.
Sift in the flour, add the bicarb of soda, baking powder, milk and treacle, and beat well to get a smooth batter.
Butter a baking dish (mine is 27x21 cm, 11 x 8.5 inches), and pour the batter in, levelling it well.
Bake for 30 minutes until the sponge is well risen, and a skewer inserted in the middle comes out clean.
Remove from the oven, and allow it to cool for 10-15 minutes.
To make the sauce, cut the butter into cubes, and add it to a pan set over a low to medium heat.
Add the light brown sugar, and mix so the butter can melt.
Pour in the cream, treacle and vanilla extract, and whisk well to get a smooth sauce.
Once it comes to a boil, let it bubble up for a minute, then remove it from the heat.
Pour half of the sauce over the warm sponge, and reserve the rest of the sauce of serving with the cake.