Leave the butter and eggs at room temperature. Weigh out the other ingredients to be ready to use. The ingredients are for the topping. Combine the sugar and the cinnamon (optional.)
Cream the softened butter and then add the sugar.
Add the beaten egg to Step 2 in 2-3 batches while mixing it in well. Then add the salt.
Sift the flour and baking powder together and then add 1/3 of it to Step 3 and mix it in.
Also add 1/3 of the milk and stir. Alternate adding the remaining powder ingredients with the milk for two more turns, mixing them in each time.
Add the blackberries to Step 5 and stir them in. (If you will be topping the muffins with blackberries, set aside 20-30 berries.)
Pour the batter from Step 6 into the muffin tin (about 80% full).
[This Step is optional] Top the muffins with the blackberries you set aside and the combined cinnamon and sugar ingredients.
Bake for 20 minutes at 175C and they're done. Once a toothpick is removed cleanly, they're done.
These are stuffed with blackberries. If you think there doesn't need to be so many berries in the muffins, decrease the amount.
The cake is so moist that they can still be enjoyed 2 days later.