Preheat oven to 350°F and generously grease a 12 cup muffin tin with butter.
Prepare the raspberry jam: In a small bowl combine 1 tsp water with cornstarch and set aside. Add raspberries, sugar and remaining water in a small saucepan over medium heat. Stir the mixture as it begins to cook, breaking up some of the raspberries. Once simmering, add the cornstarch mixture and simmer for another 3 minutes. Remove the pan from heat and press through a fine mesh strainer to remove the seeds. Allow to cool completely before using.
Prepare the shortbread cookie dough: In a large bowl add butter, sugar, vanilla extract, flour and salt. Beat until the dought starts to come together, but still crumbly, about 2 minutes.
Assemble the cookies: Press 2-3 tbsp of crumble cookie dough into each muffin cup and pat it down with a small measuring cup or your fingers. Reserve the rest of the dough crumbles. Pour about 1 tbsp raspberry jam into each cup and spread all over the dough. Sprinkle the rest of the dough crumbles over the raspberry jam, leaving some spots of jam to peak through.
Bake for 20-22 minutes or until slightly golden-brown and transfer the muffin tin to a cooling rack. Allow to cool in the pan for 10 minutes. Loosen the edges with a butter knife, pop them out of the muffin tin and place them on a cooling rack. Dust with powdered sugar if desired.