Melt the butter in a small saucepan over medium heat. Keep cooking, swirling the pan often, for 4-5 minutes until it smells nutty and golden brown specks appear on the bottom. Pour into a bowl and cool for 10 minutes.
Preheat the oven to 325°F (165°C). Grease a 9-inch springform pan and line the bottom with parchment paper. In a large bowl, whisk together the almond flour, coconut flour, golden sweetener, baking powder, cinnamon and salt.
Add the cooled brown butter, eggs, sour cream, almond milk, banana extract and vanilla extract to the dry ingredients. Mix until a smooth, thick batter forms and spread it evenly in the pan.
Bake for 30-35 minutes, until the top is deep golden and a toothpick inserted in the center comes out clean. Cool in the pan for 20 minutes, then release the sides and cool completely.
For the banana cream, beat the cream cheese and powdered sweetener until completely smooth. With the mixer on low, gradually pour in the cold heavy cream, banana extract and vanilla extract.Increase the speed to high and whip for 2-3 minutes until thick, stiff peaks form.
Spread the banana cream over the top of the cooled cake in a thick layer with generous swirls. Sprinkle with the toasted pecans and a light dusting of cinnamon, then chill for at least 1 hour before serving.