Preheat the oven to 375°F. Line two baking sheets with parchment paper.

In a medium bowl, whisk together the oats, flour, milk powder, baking soda, salt, cinnamon, ginger, nutmeg and allspice.

Add the dark brown sugar and granulated sugar to the cooled brown butter. Beat with an electric mixer until very light and fluffy.

Add the maple syrup, egg, vanilla and maple extract, if using. Beat again until the mixture is light and fluffy, about 1–2 minutes.

Add the oat mixture and mix just until everything is combined. Do not overmix.

Scoop the dough into roughly 2-tablespoon portions and place them on the prepared baking sheets, leaving space between each cookie.

Bake for 8–10 minutes, until the edges are set and lightly golden and the centres still look slightly soft.

Let the cookies cool on the baking sheets for about 5 minutes, then transfer them to a wire rack to cool completely.

Combine all of the glaze ingredients in a small bowl and whisk until smooth, adding enough milk to make it easily drizzleable.

Drizzle the glaze over the cooled cookies and let it set before serving.