Preheat the oven to 180°C and butter and line the base of a 20cm springform tin.
Wash the rhubarb and chop most of it into 1cm pieces. Leave a few larger pieces for the top. If you are adding strawberries, make sure they are thawed and not too large.
Place the butter and sugar and essences into the Thermomix bowl and chop 5 sec/speed 5. You may need to chop the butter a little longer than suggested before adding the butterfly if the butter is colder.
Insert the butterfly and whip 1 min/butterfly/speed 4.
Scrape down the sides of the bowl and repeat. You cannot afford the butter to melt, so make sure it is aerating and not melting. Stop whipping if it looks like the butter is beginning to melt and proceed with the recipe.
Remove the butterfly. Add the eggs one at a time and beat 30 sec/speed 4. Scrape down lid and sides of bowl in between the eggs.
Add the flour, baking powder, milk and salt and whip 20 sec/speed 6. Finish combining with a spatula if necessary.
Stir in about half of the fruit, folding it into the batter. Spread the batter into the prepared cake tin. Finish it with the larger pieces of fruit then sprinkle with the raw sugar.
Bake for around 1 hour or until a skewer test is nearly clean. A little gooey in the middle is ok and a little wobble.
Cool in the tin and remove and slice to taste, serving with cream, sour cream, ice cream, custard...dusted generously with icing sugar as well!