Make the cupcakes according to package instructions. Lightly grease the muffin tin and fill the batter almost to the rim of each muffin cup. Bake according to package directions and let cool.
In a food processor combine the mascarpone cheese, strawberries, 1/4 cup sugar, and lemon juice. Process the mixture until smooth. Transfer the strawberry mixture to a pastry bag fitted with a small tip. Push the tip gently into the bottom of a cupcake and squeeze in the strawberry mixture until the cupcake plumps. Continue with the remaining cupcakes.
In a small saucepan bring the 1/2 cup sugar, water, and vanilla extract to a simmer over medium heat. Simmer until the sugar is dissolved. In a medium bowl combine the vanilla syrup with the confectioner's sugar. Stir until smooth. Working quickly, use a rubber spatula spread the icing on the top of the cupcakes. If the icing begins to firm up, place in the microwave for a few seconds to warm up again.