Preheat the oven to 180°C (350°F) and line a muffin tin with 12 paper liners.
In a mixing bowl, whisk together canola oil, brown sugar, and vanilla extract until well combined.
Pour in the Guinness or stout beer and mix until the ingredients are fully combined.
In a separate bowl, sift together the flour, cocoa powder, and baking powder.
Slowly pour the dry ingredients into the wet ingredients, mixing until the batter is smooth.
Divide the batter evenly among the prepared paper liners.
Bake for 18-20 minutes or until a toothpick inserted into the centre of a cupcake comes out clean.
Remove the cupcakes from the oven and let them cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.
Make the buttercream: Using a hand mixer or a stand mixer with the paddle attachment fitted, beat dairy-free butter and vanilla for 1-2 minutes until creamy.
Add in half of the icing sugar and beat on medium until combined. Add in the cocoa powder and remaining icing sugar and continue to mix. As the mixture thickens, add in the Guinness or stout beer a tablespoon at a time until the frosting is light and creamy.
Once the cupcakes are cool, add the buttercream to a piping bag with your favourite tip and pipe swirls on top of each cupcake.