Preheat oven to 400°F. Lightly grease or line a 12 cup muffin tin with paper muffin cups.
In a large mixing bowl, measure and combine flour, baking powder and soda, cinnamon and salt.
Using an electric hand mixer or stand mixer, beat butter for about 1 minute, then add sugar and continue beating for another 2-3 minutes until you have a creamy texture. Add egg and yogurt and beat just until well combined.
Measure milk in a liquid measuring cup, and add vanilla.
On the lowest speed, start adding the flour to the butter mixture, and alternating with the milk mixture, and ending with flour. Once combined, remove and if there is any flour at the bottom of the bowl, mix by hand and it’s important to not overmix.
Using an ice cream scooper, add about 2/3 's to 3/4's of the batter to the muffin cups, then add a teaspoon of the plum jam, then add the remaining batter. Some of the jam may be exposed.
Bake muffins for 17-20 minutes ,until golden in colour or a toothpick insert comes out clean.
Topping: mix together using a fork the flour, sugar, cinnamon and break butter into pieces then add to mixture. Sprinkle lightly on top of each muffin. You may need to flatten the batter to allow the topping to sit on top of the muffins.
Let cool in pan for about 10 minutes. Serve warm or allow them to cool further on a wire rack.