Preheat the oven to 350°F (180°C) and position the rack in the middle of the oven.
Spray a 9-by-2-inch (23-by-5-centimeter) round cake pan with nonstick olive oil cooking spray.
In a large bowl, whisk together the rice flour, glutinous rice flour, sugar, cocoa powder, baking powder, and salt.
In a medium bowl, whisk together the eggs, coconut milk, sweetened condensed milk, and vanilla. Set aside.
In a small saucepan over medium-low heat, melt the chocolate and butter together. Gently stir until the mixture is completely smooth.
Pour the chocolate mixture into the wet ingredients and whisk until incorporated and smooth.
Pour the wet ingredients into the dry ingredients and whisk until the batter is smooth.
Pour the batter into the prepared cake pan and bake for 1 hour, or until a wooden skewer inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes. Remove the cake from the pan and transfer to a wire cooling rack with a baking sheet underneath.
Let cool completely, then transfer the cake to a cake stand. Slice and serve with a generous scoop of chocolate mousse and a dusting of confectioners’ sugar, if desired.
Wrap leftovers tightly in plastic wrap or store in an airtight container at room temperature for up to 3 days.
