Prep: Spray a 9x9 pan with nonstick spray and line with parchment paper for easy removal later.

Make the Peach Filling: Add diced peaches, sugar, lemon juice, cinnamon, and nutmeg to a medium sauce pan over medium heat. Cook for 4-6 minutes. It will be quite bubbly. Stir the corn starch and water together in a small bowl and stir into the peach mixture. Cook for about 1 minute, then remove from the heat immediately and stir in the vanilla. The juices should be very thick. Taste and see if it needs an extra tbsp of sugar or so - if your peaches aren't super flavorful, you may want to stir in this extra bit.Transfer filling to a medium bowl to cool completely in the fridge.Make-Ahead Tip: This can be done up to 2 days ahead. Store in a jar in the fridge.

Make the Bars: Preheat your oven to 350℉. Melt the butter in a medium bowl. Stir in the remaining bar ingredients. Reserve 1 cup of the mixture for the topping. Press the rest into your prepared pan. Bake for 10 minutes. Evenly spread the cooled peach filling over the crust. Sprinkle (or press small clumps flat in your hand to dot over the top of the bars) the reserved crumble mixture over the top of the peaches. Return to the oven and bake for 28 to 33 minutes – the topping will be golden brown and the bars will only have a slight jiggle when you move the pan. Let the bars cool to room temperature, then refrigerate for at least 2 hours before adding the glaze and slicing.

Glaze: If using, whisk together all glaze ingredients, and adjust to a thinner or thicker consistency as desired, by adding a splash more milk or a spoonful more powdered sugar.

Serve + Store: Slice bars and enjoy! Store bars in an airtight container in the fridge for 2-3 days. The fresh fruit filling means they do need to be refrigerated.