In a bowl, whisk the eggs and brown sugar together for about 5 minutes until the mixture is light, fluffy, and has tripled in volume.
Gently fold in the flour, baking powder, baking soda, and a pinch of salt into the whipped egg mixture.
Pour in the melted butter and crème fraîche, along with a little vanilla extract, and mix on low speed until just combined.
Transfer the batter to a lined round cake pan and smooth the top.
Bake the cake in a moderate oven for 25 to 30 minutes, checking for doneness around the 28-minute mark.
While the cake is baking, place the strawberries on a baking sheet, sprinkle them with sumac, and roast for about 15 minutes until they are softened.
Once the cake is baked, allow it to cool completely before removing it from the pan.
In a mixing bowl, combine the double cream (or heavy whipping cream) with extra brown sugar and a splash of vanilla extract, then whip until soft peaks form.
Spread the whipped brown-sugar cream over the cooled cake.
Top the cream with the roasted sumac strawberries, and if desired, drizzle with a little extra vanilla or brown sugar.
Serve the cake at room temperature.
