Make the filling

• Remove the pits from the peaches. To do this, slice the peach in half around the pit. Then twist to separate the halves. Use a melon baller or a small spoon to gently remove the pit. It's OK if you accidentally crush the peaches, since the filling will be cooked anyway.
  600 g peaches
• Cut the peaches into cubes that are about ½ inch. Place all of the peaches in a large bowl.
• If using Instant ClearJel, mix it with the sugar in a separate, small bowl.
  25 g cornstarch, 100 g sugar
• Add the sugar, cornstarch (or ClearJel), lemon juice, cinnamon, salt, and vanilla extract to the peaches in the large bowl. Gently fold to mix all the ingredients. Set aside until needed.
  100 g sugar, 25 g cornstarch, 30 mL lemon juice, 1 tsp ground cinnamon, Pinch of salt, 10 mL vanilla extract

Making the base and topping

Optional step to make brown butter

• This step is optional, but I like to use brown butter whenever I can, since it adds an incredible layer of flavor. If you choose to make brown butter, do this before making the filling or even the day before so that it has time to cool down.
• Place the butter in a small saucepan and heat over medium heat until the butter melts. Add the milk powder and whisk to dissolve the milk powder. Keep heating the butter to bring it to a boil while frequently whisking.
  226 g unsalted butter, 2 tbsp milk powder
• When the butter starts to boil, the milk solids will separate and sink to the bottom. Whisk the mixture to prevent the milk solids from caramelizing too fast and unevenly.
• The milk solids will start to turn a darker color, going from a blonde color to a golden brown. I prefer to cook the milk solids until it turns a light amber color.
• When the brown butter turns a dark golden or light amber color, transfer the butter and milk solids into a separate heat-proof container. Let it cool down until it’s only slightly warm.

Making the dough for the base and topping

• Butter the sides and bottom of a 9 x 13 inch baking pan, and line it with parchment paper. Ideally with an overhang so that you can easily remove the crumble bars from the pan once baked. Set aside.
• Preheat the oven to 350°F / 180°C.
• In a large bowl, place the flour, sugar, and salt. Whisk to combine. If you’re not making brown butter, you can leave out the milk powder, or add it to the flour mixture instead.
  400 g all purpose flour, 200 g sugar, ¼ tsp sea salt
• If not using brown butter - Melt the butter until it's softened, or just melted. Make sure the butter is at room temperature or just a little warm. Whisk in the egg yolk and vanilla.
  1 egg yolk, 1 tsp vanilla
• If using brown butter - whisk in the egg yolk, vanilla, and 2 tbsp water (or bourbon) to the warm brown butter.
  1 egg yolk, 1 tsp vanilla
• Add the butter mixture, a little at a time, to the flour mixture while stirring. The flour mixture should absorb the butter and start to form clumps.
• Once all the butter mixture is added, you will end up with a flour mixture that looks like clumps and is crumbly. When pressed together in your hand, the clumps will stick together like a dough.

Assembling the peach crumble bars

• Transfer about ¾ of the dough crumbles into the prepared baking tray. Flatten the crumbles into the bottom of the baking pan. Make sure that is layer has an even thickness, and the crumbles are pressed well into the bottom, like a cheesecake crust.
• Add the oats or sliced almonds into the 25% of the dough that is remaining. This will be the crumble topping.
  ¼ cup rolled oats or sliced almonds
• Once the base has been pressed into the pan, scrape out all of the peaches from the bowl into the baking tray, on top of the shortbread base.
• Spread out the peaches to form an even layer that covers the whole surface of the base.
• Sprinkle the remaining dough with the oats or almonds on top of the peach filling to create the crumble topping.

Baking

• Place the baking tray in the middle rack of the oven and bake for about 50 - 60 minutes. If the crumble topping is burning, cover the top with foil.
• Once baked, the crumble bar topping should look toasty (but not burnt), and the filling will be thick and a little bubbly. Remove from the oven and let it sit for a few minutes to cool down slightly.
• Gently remove the peach bar from the baking tray while it's still a little warm so that it can cool down on a wire rack. The peach bar will be very soft and flimsy and will break easily too, so use a cutting board underneath to help move it.
• Let the peach crumble bar slab cool down to room temperature. Or chill it in the fridge. I prefer to chill these bars in the fridge overnight.
• Once the bars have cooled down or have been chilled, cut the slab into squares (or any size you like).
• Serve at room temperature or chilled.