Pre-heat the oven to 170°C and line and grease a 10” square baking tin.
Whisk together the ground almonds, gluten-free flour, baking powder, bicarbonate of soda and the sugars until fully mixed.
In a separate bowl whisk together the sour cream, eggs and vanilla extract then place to one side.
Place the butter in a saucepan and heat over a medium flame. The butter will melt. Listen carefully and the butter will start hissing and cracking and forming little brown bits at the bottom of the pan. You want to wait until the noises start to subside, the butter smells toasty and is turning a darker colour. Remove from the heat as soon as it’s ready so it doesn’t begin to burn.
Pour the brown butter in a slow and steady stream into the sour cream and egg mixture, whisking in all the while so that it doesn’t start to cook the eggs.
Once all the butter is incorporated, pour the wet ingredients into the dry ingredients and mix in together.
Pour the batter into the cake tin, then arrange the plums on top, cut side up. Drizzle over the honey and scatter the almond flakes over evenly.
Bake in the oven for about 30 minutes until the top is lovely and golden.