Preheat oven to 350°F. Grease a 9x13 baking dish and set aside.
In a medium bowl, whisk the flour, baking powder, baking soda, and salt together. Set aside.
2 ½ cups (370g) gluten free flour blend, 2 teaspoons baking powder, ½ teaspoon baking soda, ½ teaspoon salt
Using a hand mixer or a stand mixer fitted with the paddle attachment, cream the butter with the sugar until smooth. Mix in the mashed bananas, milk, yogurt, and vanilla extract. Add the dry ingredients to the wet ingredients and mix on low speed until combined.
1 cup (226g) unsalted butter, 1 ⅓ cups (267g) granulated sugar, 3 large ripe bananas, 1 cup (240ml) milk, ¾ cup (168g) Greek yogurt, 2 teaspoons vanilla extract
Spread the batter into the prepared pan. Bake for 35-40 minutes, or until a toothpick inserted into the center of the cake comes out clean. Cool completely before frosting.
For the chocolate buttercream:
Using a hand mixer or a stand mixer fitted with the paddle attachment, beat the butter until smooth and creamy, about 5-6 minutes.
1 cup (226g) unsalted butter
Add the powdered sugar, cocoa powder, and salt and beat to combine. Add the milk and beat for another 5 minutes.
2 ½ cups (300g) powdered sugar, ¾ cup (63g) unsweetened cocoa powder, ⅛ teaspoon fine sea salt, ¼ cup (60ml) whole milk