Step 1

Melt the butter in a small pot on the stove or a microwave-safe, bowl. Allow to cool for a few minutes.

Step 2

In the meantime combine the Gluten Free Flour, light brown sugar, ground ginger and kosher salt in a mixing bowl and whisk together.

Step 3

Using a fork or a small spatula, lightly mix the dry ingredients and the melted butter together until crumbles form. Don’t overmix.

Step 4

Place the crumble topping in the fridge while you prepare the gluten free rhubarb cake batter

Step 5

Wash the rhubarb stalks under cold water and dry them with a paper towel. Remove both ends and any leaves as they are toxic.

Step 6

Slice the rhubarb stalks into rounds around ¼" wide round slices (kinda like cutting celery) and add them to a mixing bowl. If the stalks are particularly thick, you might want to cut them lengthwise first and then into smaller pieces to ensure even cooking.

Step 7

Add the sliced rhubarb to a small bowl, add the gluten free flour, and stir to combine making sure the fruit is coated with the flour. Don't add the raspberries since they don't have to be coated in flour.

Step 8

Set the fruit aside while you prepare the rhubarb coffee cake batter.

Step 9

Preheat the oven to 350F.

Step 10

Spray the 9x13 baking pan with cooking spray and line it with parchment paper and set it aside.

Step 11

In a small bowl combine the gluten free flour, almond flour, kosher salt, and baking powder.

Step 12

To a mixing bowl add the granulated sugar and the lemon zest. Remember to reserve 20 grams of sugar for the egg whites. With the back of a spoon or clean hands rub the zest into the sugar.

Step 13

Carefully separate the eggs. Add the egg yolks to the bowl with the lemon sugar and place the 4 egg whites in a clean, separate mixing bowl. Set the egg whites aside.

Step 14

Whisk the lemon sugar and egg yolks together until lighter in color and doubled in volume. At first, the mixture will thicken up but if you keep whisking it will turn into a smooth liquid.

Step 15

Add the oil and milk (or water) and vanilla to the egg yolk mixture and whisk until smooth. Set aside while you whisk the egg whites.

Step 16

With an electric hand mixer (or stand mixer) whisk the egg whites with a pinch of salt until they appear frothy. Add the reserved 20 grams of granulated sugar little by little and mix until the egg whites appear glossy. The egg whites should be soft/medium stiff. Overmixed egg whites appear dull and dry.

Step 17

Add the dry ingredients to the wet ingredients in two parts. Whisk the ingredients together until the batter is smooth.

Step 18

To the batter, add about ⅓ of the beaten egg whites and fold them in gently. This will help to loosen up the batter. Then, add the remaining egg whites and fold them in carefully until the rhubarb coffee cake batter becomes light and airy. Make sure to be gentle and not overmix the batter.

Step 19

Transfer the gluten free rhubarb cake batter to the prepared baking dish. With a small offset knife or the back of a spoon, spread it out evenly.

Step 20

Sprinkle the cake in an even layer with the prepared rhubarb and raspberries. There is no need to press the fruit into the batter.

Step 21

Top the Rhabarber Streuselkuchen with the prepared brown sugar streusel. It's okay if some of the fruit peaks through.

Step 22

Bake the Gluten Free Rhubarb Coffee Cake at 350F for 34-38 minutes. The cake will be golden brown and a cake tester or toothpick insert in the center comes out clean. If you prefer using a digital thermometer, ensure the internal temperature in the center of the cake is above 200F.

Step 23

Remove the coffee cake from the oven and place it on a cooling rack. Allow for it to cool in the pan for at least 15-20 minutes before serving.