Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper to make cleanup easier.
In a large bowl, cream together the softened butter and granulated sugar until the mixture becomes light and fluffy.
Beat the egg into the butter and sugar mixture until fully combined.
In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add this dry mixture into the wet ingredients, stirring until just combined.
Stir in the muesli, making sure it’s evenly distributed throughout the dough.
Scoop spoonfuls of the dough onto the prepared baking sheet, spacing them about 2 inches apart. You can flatten them slightly with the back of a spoon if you want a thinner cookie.
Sprinkle a little demerara sugar on top of each cookie for a nice crunch.
Bake the cookies for 12-15 minutes, or until the edges are golden and the center is set.
Let the cookies cool on the baking sheet for a few minutes, then transfer them to a wire rack to cool completely.