Preheat oven to 325°. Line 16 muffin cups with liners. Beat butter and sugar with a stand mixer until creamy. Add eggs, vanilla, and buttermilk; beat until well blended.
Whisk together flour, baking powder, baking soda, and salt in a bowl. Gradually beat into butter mixture until well blended.
Whisk together jam and rose water to taste in a small bowl. Spread 2 heaping tbsp. batter in each liner. Top with 1 tsp. jam mixture, then another 2 heaping tbsp. batter, carefully spreading it to cover jam.
Bake cupcakes until a toothpick inserted toward the edge comes out clean, 25 to 27 minutes. Let cool in pans 5 minutes, then transfer to racks to cool completely. Spread with frosting and decorate with rose petals.