Preheat the oven to 350°F (175°C). Line an 8-inch (20-cm) square baking pan with parchment paper. In a small bowl, whisk together the tapioca starch, flour, black cocoa powder, sugar, salt, and baking soda.
Melt 1 ¼ cups of chocolate chips in a double boiler or heatproof bowl set over a saucepan of simmering water, stirring until smooth.
Mix the soy milk, apple cider vinegar, vegan butter, and vanilla into the bowl of melted chocolate.
Add the wet ingredients into the bowl of dry ingredients and mix until just combined. Fold in the remaining 1 cup of unmelted chocolate chips.
Transfer the batter to the prepared pan and spread evenly. Bake for 30–32 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs. Let cool completely in the pan.
To prepare the frosting, beat the softened vegan butter until smooth. Add the plant-based milk, powdered sugar, and vanilla extract. Beat until light and fluffy.
Cut the cooled brownies into 8 rectangles.
Transfer the frosting to a piping bag fitted with a small flat or round tip.
Pipe two small dots of frosting onto the brownies and press 2 candy eyes firmly onto each dot. I think it looks cute when some of the eyes are slightly different heights, or a little off center, but you can make them as you like.
Pipe diagonal lines across each brownie to resemble mummy bandages. Refrigerate for 10 minutes to help the frosting set. Serve at room temperature.