In a bowl, sift together the teff, sweet rice, oat, and tapioca flours with the baking powder, baking soda, and salt.

Separately, stir together the dairy-free yogurt and plant milk and set aside.

In a large bowl with a wooden spoon (or in the bowl of a stand mixer fitted with the paddle attachment), beat together the vegan butter and brown sugar on medium-high speed until light and fluffy, a few minutes. Beat in the egg yolks one at a time, then stir in the vanilla.

Stir about one-third of the flour into the butter mixture, then stir in half of the yogurt mixture. Repeat, ending with the remaining flour mixture. Stir in the mashed banana.

In the clean bowl of a stand mixer fitted with the whip attachment (or in a large bowl with an electric egg beater or whisk) whip the egg whites until foamy. Slowly add in the granulated sugar and whip on medium-high until the whites just hold firm peaks. Fold one-third of the egg whites into the batter, then gently fold in the remaining two-thirds until no streaks remain. Gently pour the batter into the prepared pan and spread into an even layer.

Bake the cake until a toothpick inserted near the center comes out clean or with moist crumbs, 25 to 35 minutes. Let cool completely in the pan.

Swirl the cooled ganache over the cake and let cool or chill until firm enough to slice. Lift the cake from the pan, peel away the parchment, and cut into 12 pieces. Serve at room temperature.