Beat butter and sugar with an electric mixer until smooth. Add egg and vanilla, mixing well until combine, about 2 minutes. Stir in flour, baking powder, salt, and 1 1/2 teaspoons cinnamon with the mixer on low speed. Mix until dough comes together. Add in milk. If dough is still crumbly, add milk 1 tablespoon at a time until it comes together. Fold in blueberries gently with a spatula – it is okay if they break a bit. Refrigerate for 30 minutes.