Preheat the oven to 180°C. Line a (approx.) 20x30 cm baking tray with parchment paper.
If the jam is thick, heat it in a small pan until it becomes easily spreadable. Set aside to cool.
Mix the soft butter with the sugar until light and fluffy. Add the eggs, one by one, beating them into the batter before adding the next one.
Stir in the jam, vanilla, cinnamon, salt and finally the flour mixed with the baking powder. Combine until you a get smooth, thick batter.
Transfer the batter into the baking tray. Spoon the jam over the batter, spreading it out evenly.
Bake for 30 minutes until it’s golden brown. Test it with a skewer - if it's cooked through properly, take it out of the oven and let it cool for 5 minutes in the tin, then transfer it to a wire-rack.
Let it cool completely, then cut the cake into slices.
Dust with icing sugar, just before serving.