In a small pan, combine all of the ingredients for the apple filling. Cook over low-medium heat until tender. Set aside to cool.
In a large bowl, beat the butter for the muffins until smooth. Add the sugar and continue beating until smooth and lump free (make sure there are no large lumps, but don't worry if it looks a little grainy because of the sugar). One at a time, add the eggs, beating until just combined. Add the flour and cinnamon, and fold in using a spatula until a thick paste forms. Add the milk and vanilla extract, and stir gently with your spatula until just combined. Pour the batter into a muffin pan lined with 6 large muffin liners.
Spoon the apple filling over the top of each muffin, and drizzle with slivered almonds. Bake at 350F/175C for 20-23 minutes, or until a toothpick inserted in the center comes out clean.
While the muffins bake, combine all the streusel ingredients in a small bowl. Using a pastry/dough blender (or knife and fork) cut in the butter until combined but still chunky.
Spoon the streusel over the muffins immediately after removing them from the oven.