Preheat the oven to 350°F (175°C). Lightly coat a 9-inch (23cm) square cake or brownie pan with canola oil spray, then line the bottom and sides with parchment paper, leaving extra hanging over the sides.
To make your flax egg, whisk together the flaxseed and oat milk in a small bowl until a smooth paste forms. If there are any lumps, push a small rubber spatula against the side of the bowl to break them up. Let sit for 10 minutes to bloom and thicken.
In a stand mixer fitted with the paddle, beat the butter with the sugar on medium speed until smooth and paler, 5 minutes. Add the peanut butter, vanilla, and flax egg and beat until everything is well incorporated, 2 minutes.
Add the baking powder, salt and millet flour and beat on medium speed until no dry patches remain, 1 minute.
Add the chocolate chips and gently incorporate until just mixed.
Scrape the batter into the lined pan and spread it evenly with a rubber spatula. Lightly compress the batter. Sprinkle evenly with the peanuts. Bake until the blondie is golden brown, 25 to 30 minutes. Let the blondie cool in the pan on a cooling rack for at least 1 hour or overnight before cutting.
Notes
The blondies are best eaten at room temperature. Store the blondies in an airtight container in the fridge for up to 5 days.