In a large mixing bowl, whisk together the dry ingredients: flour, baking powder, baking soda, salt, and nutmeg.

In a separate bowl, mix together the wet ingredients: melted butter, sugar, eggs, buttermilk, and vanilla until smooth.

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

Preheat your oven to 350°F (175°C). Grease and flour a baking pan or line it with parchment paper.

Transfer the batter into the prepared baking pan, smoothing it out evenly.

Bake for 20-25 minutes, until the edges are golden and a toothpick comes out clean.

Let the Maple Donut Bars cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

Sift powdered sugar into a bowl, whisk in maple syrup, milk, melted butter, and a pinch of salt until smooth.

Once cooled, pour the glaze evenly over the bars using a spatula.

Sprinkle on optional toppings immediately after glazing if desired.

Allow the glaze to set for 30-60 minutes before serving.