Preheat your oven to 350°F (175°C) and prepare a baking pan by lining it with parchment paper or greasing it.

In a medium bowl, combine the flour, baking soda, baking powder, salt, and nutmeg. Whisk until blended.

In a large bowl, cream the butter and sugar until light and fluffy, about 3–5 minutes.

Add the egg, vanilla extract, and buttermilk to the mixture and beat until combined.

Gradually add the dry ingredients to the wet mixture, stirring gently until just combined.

Pour the batter into the prepared baking pan and spread it evenly.

Bake for 18–20 minutes, checking for doneness with a toothpick.

Cool the bars in the pan for 10 minutes, then transfer to a wire rack.

Whisk together the glaze ingredients until smooth, adjusting with more milk as necessary.

Once cool, glaze the bars and allow to set before slicing.