Mix the flour (120 grams) and baking powder (¾ teaspoon) together in a small bowl and set aside.
In your stand mixer fitted with the paddle, beat the 3 eggs. Add the caster sugar (250 grams) a bit at a time to avoid it all flying away. Once all added, run on medium-high speed until doubled in volume, 8-10 minutes. While it's mixing, attend to your butter and chocolate.
When the chocolate is fully melted into the butter, remove from the heat and whisk in the the Dutch-process cocoa powder (25 grams) a bit at a time to avoid getting it all over your kitchen. Fully incorporate.
Once your eggs and sugar are ready -- doubled in size, kinda pale -- add the canola oil (1 tablespoon) and vanilla (1 teaspoon) and mix on low to combine.
Remove the bowl from the mixer and add all the butter/chocolate mixture into the bowl. (How I know that trying to add the chocolate to the mixing bowl while still in the mixer will not turn out well is not important.) Return the bowl to the mixer and mix on low until fully incorporated -- it'll get darker and darker as the chocolate mixture is blended in.
Add the flour and baking powder slowly and mix well to combine.
Remove the bowl from the mixer and try to get as much of the batter off the paddle as you can. Scrape down the sides.
Cover and let sit at room temperature for 5-10 minutes.
Heat oven to 350°F / 180°C. Put rack in the middle of the oven.
Prepare two sheet pans by adding parchment paper.