Preheat the oven to 180°C/gas mark 4. Grease a 23cm square cake tin and line with baking parchment.
Trim the rhubarb and chop into 3cm pieces. Place in a bowl with the lemon juice. In a separate bowl, beat together the flour, butter, sugar, eggs and vanilla extract. Fold in half the rhubarb and spoon the mixture into the bottom of the tin, spreading it out with a spatula. Scatter over the remaining rhubarb.
For the topping, rub the butter into the flour, then stir in the sugar and ginger. Sprinkle over the cake mixture and bake for 40-50 minutes. Leave to cool for 10 minutes, then remove from the tin. Dust with icing sugar, then slice into squares. Serve warm or cold with crème fraîche, drizzled with honey.