Mix butter and sugars together. Add egg and vanilla, beating for another 30 seconds.
Before adding to mixing bowl, mix flour, baking soda and salt together. Sift flour mixture into baking bowl a bit at a time, then beat on low for 1-2 minutes.
Stir in chocolate chips and muesli by hand, then form dough into spoonfuls and drop of parchment-lined baking sheets.
Bake for 10-12 minutes. remove from oven and let cool for 2-3 minutes before enjoying.
