Aunt Anne's Coffee Cake

Streusel Topping:

• ⅔ cup white sugar
• ¼ cup all-purpose flour
• 1 teaspoon ground cinnamon
• ¼ cup cold butter

Cake:

• 2 cups all-purpose flour
• ¾ cup white sugar
• 2 teaspoons baking powder
• ½ teaspoon salt
• ½ cup cold butter
• 1 egg
• ¾ cup milk, or as needed
• 1 ½ teaspoons vanilla extract