Place the raspberries and sugar into a saucepan over medium heat, and bring to the boil.
Preheat the oven to 180C/350F/Gas 4. Grease a 9x5-inch loaf pan and line with parchment paper. Set aside.
Whisk together the flour, baking powder, and salt. Set aside.
Using a handheld or stand mixer fitted with the paddle attachment, beat the butter until light and fluffy. Add the sugar and beat until combined. Mix in the eggs, one at a time, beating well after each addition.
Pour half of the batter into the prepared pan, spread out, and drop spoonfuls of the raspberry sauce on top. Using a toothpick or knife, swirl the two mixtures together.
Bake for 50 - 60 minutes, or until a toothpick inserted into the centre comes out clean. Cover loosely with aluminium foil halfway through baking to prevent the cake from browning too much.
Leftover cake can be stored at room temperature or in the fridge for up to 5 days. This cake also freezes well for up to 3 months. Thaw overnight in the fridge before serving.
*For seedless raspberry sauce, puree 2 cups (250g) of fresh raspberries until smooth. Push the mixture through a sieve into a saucepan. Discard the seeds. Add 4 tablespoons of caster/granulated sugar, and bring to the boil over medium heat. Cook on medium-low heat, stirring frequently, for about 10 minutes until the sauce has thickened. Allow to cool completely.
**Don't use lemon juice in place of lemon extract because it's not strong enough, and the acidity can alter the overall results.
The calories are an estimate only.