Preheat your oven to 180°C (350°F). Grease and line the baking pan with parchment paper for easy removal.
In a mixing bowl, cream together the softened butter and granulated sugar until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, mixing well after each addition, then stir in the milk.
In a separate bowl, whisk together the flour, baking powder, ground cinnamon, and salt.
Gradually add the dry ingredients to the wet mixture, stirring until just combined. Be careful not to overmix.
Pour the batter into the prepared baking pan, smoothing the top with a spatula.
Gently dollop the fruit jam over the batter and use a knife to swirl it into the top layer for a marbled effect.
Bake in the preheated oven for 30 minutes, or until a toothpick inserted into the center comes out clean – the edges should be lightly golden.
Remove from the oven and allow to cool in the pan for 15 minutes before transferring to a wire rack to cool completely.
Once cooled, cut into neat squares and serve with the optional garnish of fresh fruit and powdered sugar, if desired.
