Combine the cornstarch and 2 tablespoons of the water in a deep microwave-safe bowl and stir to dissolve the starch.
Add the remaining 6 tablespoons water, stir well, and cook for 1 minute, or until the mixture is slightly gelatinous, thickened, and cloudy.
Alternatively, do this on the stove in a small saucepan, until the same results are achieved, 1 to 2 minutes.
Let cool completely before using.
Whisk the cornstarch mixture, milk, sugar, and salt in the bowl of a stand mixer fitted with a dough hook.
Add the flour and yeast on top.
Mix on medium speed for 2 minutes, starting the countdown and raising the speed to medium-high once the ingredients are beginning to combine.
Slowly add the butter while the mixer is on.
Once all the butter is in, mix on medium-high speed for 4 minutes, stopping to push the butter down with a spatula if it sticks to the sides of the bowl. Do not add extra flour; it’s normal for the dough to look like batter.
Gather it in the center of the bowl with a rubber spatula, tightly cover the bowl with plastic wrap, and let stand for 45 minutes. This time mostly serves to ensure adequate moisture.
Use a rubber spatula to gently deflate the dough, and gather it in the center of the bowl again.
Tightly cover with plastic wrap again, and refrigerate for 18 hours.
Coat an 8 x 4-inch loaf pan with spray.
Use a rubber spatula to gently deflate the stiff dough.
Place the dough in the pan and smooth the top with lightly moistened hands if needed.
Loosely cover with plastic wrap, and let rise for 1 1/2 to 2 hours, until doubled.
Preheat the oven to 400°F.
Carefully remove the plastic wrap and bake the brioche for 10 minutes.
Lower the temperature to 350°F, and bake for 15 to 20 minutes longer, until it reaches a deep golden brown color on top.
Carefully remove from the pan, transfer to a wire rack, and let cool completely.