Preheat your oven to 325 degrees
Mix your butter, sugar, eggs, milk, sourdough discard and vanilla extract using a wooden spoon or whisk in a large mixing bowl.
Add the dry ingredients including the flour, salt, baking soda, and baking powder into the bowl with your wet ingredients and stir until just combined. Sprinkle the cinnamon over the top and fold it in, so that it looks as if it’s got a ribbon of cinnamon. You can mix it through, but I like the swirls.
Grease your muffin tin, or add muffin liners to the muffin pan.
Divide the muffin batter among 12 muffin spots.
In a small bowl, take your streusel ingredients and mix by hand until the butter has broken down and mixed with the flour, sugar and cinnamon. You do not want it to be a solid dough. You want it to be a crumble. Add about 1 tbsp of the crumble topping onto the top of each muffin. Make sure not to add too much or it will sink into the muffin.
Bake the muffins in the oven from 15-20 minutes. You can check by placing a toothpick into the middle and check to see if there is any liquid batter. You want to let the top slightly brown.
Let them cool on a wire rack before eating. You can store these in an airtight container, once they have cooled completely.