In a small saucepan, combine water and cornstarch, stirring until smooth. Add raspberries and sugar to the saucepan. Cook over medium heat, stirring constantly, until the mixture thickens and the raspberries break down. Remove from heat and let it cool completely.

Preheat your oven to 350°F (175°C) and line a muffin tin with cupcake liners. In a medium bowl, whisk together flour, baking powder, baking soda, and salt. In a large bowl, beat the eggs and sugar until light and fluffy. Add melted butter, vanilla extract, lemon zest, and lemon juice to the egg mixture, mixing well. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Fill each cupcake liner halfway with the batter. Add a teaspoon of the cooled raspberry filling to the center of each. Top with more batter until the liners are about ¾ full. Bake for 18-20 minutes, or until a toothpick inserted into the cake (not the jam) comes out clean. Allow the muffins to cool completely on a wire rack.

In a chilled mixing bowl, beat the heavy whipping cream until soft peaks form. Add powdered sugar, lemon zest, and lemon juice. Continue beating until stiff peaks form. Transfer the frosting to a piping bag and frost the cooled muffins as desired.