Preheat the oven to 180C/350F/Gas 4. Line a 12-cup muffin tin with cupcake cases, and set aside.
Mix the raspberry jam with the fresh raspberries, lightly crushing them. Set aside.
Whisk together the flour, baking powder, baking soda, and salt. Set aside.
Whisk together the eggs. Add the sugar, and whisk until combined. Add the vanilla extract, vegetable oil, and sour cream, and whisk until combined. Fold in the dry ingredients.
Add the crushed raspberry mixture, and gently mix together to create a marbled appearance.
Divide the batter evenly between the muffin cups, filling only 2/3 full.
Bake for 15 - 20 minutes, or until a toothpick inserted into the centre comes out clean. Allow to cool for 5 minutes in the pan, before transferring them to a wire rack to cool completely before frosting.
For the Frosting
Using a handheld or stand mixer, beat together the icing sugar and butter until light and fluffy. Beat in the milk and raspberry jam until smooth, and spreadable.
Transfer the frosting to a piping bag, and frost the cooled cupcakes. You can also frost the cupcakes using a knife, like I have here. Garnish with fresh raspberries.
Notes
*Make your own cake flour! Measure out 1 and 1/2 cups (187g) of plain/all-purpose flour, remove 3 tablespoons, and replace with 3 tablespoons of cornflour/cornstarch. Sift well.Leftover cupcakes can be stored, covered tightly, in the fridge for up to 3 days. Frosted or unfrosted cupcakes can be frozen for up to 3 months. Thaw overnight in the fridge.Cupcakes can be made 1 day in advance, covered, and stored at room temperature. Cover the frosting, and store in the fridge until ready to use.