Preheat oven to 175°C (350F) and line a muffin tray with cupcake liners.
Sift and mix flour, cocoa powder, salt, baking powder, and baking soda in a medium-sized bowl and set aside.
Place brown and granulated sugar and vegetable oil in a stand mixer fitted with the paddle attachment and mix on medium speed for 1-2 minutes.
Add eggs, one by one, and vanilla extract and mix for another 2 minutes. Use a spatula to scrape any ingredients from the side of the bowl.
Add dry ingredients, alternating with milk in 2-3 additions. Mix on low speed until just combined. Do not over mix. Mixture will be quite runny.
Divide batter between 12-14 cupcake liners, filling until ⅔ full. Do not overfill. Bake in a preheated oven for 18-22 minutes until a skewer inserted into the center comes out clean.
Transfer to cooling rack and cool cupcake sponge completely before frosting.