Heat oven to 180C/160C fan/gas 4. Grease a deep 20cm round cake tin and line the base with baking parchment.
To make the crumble topping, put the flour, sugar and ginger in a bowl and mix together. Add the butter and rub together until the mixture resembles breadcrumbs. Stir in the oats and, using your hands, bring the mixture together into a dough, wrap in cling film and chill until needed.
Put the rhubarb pieces in a bowl with the sugar and orange zest, and mix together. Set aside while you make the cake batter.
Put the butter, sugar and orange zest in a large bowl and, using an electric whisk, beat together until light and fluffy, about 5 mins. Add the eggs, one at a time, beating together until combined before adding the next.
In a separate bowl, mix the flour, baking powder, vanilla paste and a pinch of salt together and, in two additions, fold into the butter mixture, alternating with the soured cream. Tip into your cake tin and level out with a spatula. Top with the rhubarb mixture and finish by breaking the chilled crumble into irregular-sized pieces and scattering over the rhubarb. Bake in the oven for about 1 hr 10 mins or until the crumble is golden and a skewer inserted into the middle of the cake comes out clean. Check the cake after 1 hr – if it is colouring too quickly, cover lightly with foil for the final 10 mins.
Allow to cool in the tin for 10 mins before carefully transferring onto a wire rack to cool completely. Will keep in an airtight container for up to 3 days, but the crumble will lose some of its texture after the first day.