Preheat your oven to 175C/350F. Line an 8in square cake pan with parchment paper and set aside.
In a mixing bowl, add the sweetener and butter. Cream together with a hand mixer on medium speed until smooth.
Add the eggs, banana flavoring, and vanilla extract. Mix with your hand mixer until combined.
Sift in the almond flour, cocoa powder, baking powder, and salt. Fold together with a spatula, and mix until combined.
Spoon the mixture into your prepared pan and smooth out.
Bake for 25-35 minutes, until the cake springs back when touched on the top.
Leave to cool in the pan for 10 minutes before lifting out onto a cooling rack. Set aside to cool completely.
To make the frosting, add the chocolate into a heatproof bowl, placed over a saucepan of simmering water. Heat until it is about 70% melted, then remove from the heat and continue to stir until completely melted.
Add the sour cream and flavoring and whip together using the whisk attachment on your hand mixer, until the frosting is thick and velvety.
Spread the frosting over the top of the cooled cake.