Preheat the oven to 375º F / 190º C). Line a 9×13 inch pan with foil or parchment.

Cut the cold butter into 1/2 inch cubes and set aside. In a medium bowl, stir together white and brown sugars, flour, oats, salt and baking powder. Cut in the butter and egg using a fork or a pastry cutter, or even your fingers. Dough should have a rough, sandy texture. Move half the dough into the lined pan and press into the bottom using your fingers.

In another bowl, gently stir together the blueberries, sugar, cornstarch and lime juice and zest until just combined. Sprinkle the blueberry mixture over the crust. Add almonds to the remaining dough and give it a quick stir, then crumble over the berry layer. I like to squeeze a few bits together in my fingers to ensure some nice big crumbs. Sprinkle Demerara sugar evenly over the top (if you’re using it).

Bake for 45-55 minutes, or until top is a light golden brown. Set pan on a wire rack and let it cool completely before cutting into squares (refrigerated bars cut even better). Bars will keep airtight in the fridge for up to 10 days, or in the freezer for up to 6 months.