I recommend making the jam the night before you want to make the cupcakes so that it has time to fully cool.
Add all of the jam ingredients to a large saucepan over medium-low heat. Allow the jam to simmer for about 20 minutes, stirring frequently until thickened.
Transfer the jam to a glass jar and allow to cool to room temperature. Store in the fridge for up to two weeks.
Lemon Cupcakes
Preheat the oven to 350°F and line a 12 cup pan with cupcake liners.
Whisk the cake flour, baking powder, and salt together in a large bowl.
In a separate bowl, whisk the melted butter, sugar, and lemon zest.
Add in the eggs, vanilla, and lemon juice and whisk until fully combined.
Stir in about half of the flour mixture. Then stir in the yogurt and the rest of the flour mixture and mix until fully combined. The batter will be thin.
Pour about ¼ cup of batter into each cupcake liner (they should be about halfway full).
Bake at 350°F for 18-22 minutes or until a cake tester comes out clean.
Remove from pan and let cool on wire rack. Let the lemon cupcakes cool completely before filling and frosting.
Use a cupcake corer to remove the center of each cupcake, then fill with 1-2 teaspoons of cooled raspberry jam.
Mascarpone Frosting
In a stand mixer fitted with the paddle attachment, cream together the softened butter and mascarpone cheese until completely smooth.
Then add in the powdered sugar and beat on slow until combined.
Add in 1-2 tablespoons of the raspberry jam (to taste). If desired, pass through a fine mesh strainer first to remove the seeds.
Then beat the frosting for 3-4 minutes on high speed until light and fluffy.
Frost cooled cupcakes, then garnish with fresh raspberries, lemon slices, and lemon zest.
Notes
These cupcakes will last in the fridge for up to 3 days. However, they taste best when served at room temperature! I like to let them sit at room temperature for about an hour before serving.