Preheat your oven to 180°C (160°C fan/350°F/Gas Mark 4). Grease and line a 20cm (8-inch) round cake tin with parchment paper.

Prepare the rhubarb swirl: In a small saucepan, combine the chopped rhubarb, 50g granulated sugar, and 1 tablespoon of water. Cook over medium heat for 5-7 minutes, stirring occasionally, until the rhubarb is softened but still holds its shape. Set aside to cool.

Make the cake batter: Cream together the softened butter and 175g caster sugar in a large bowl until light and fluffy. Beat in the eggs one at a time, adding a tablespoon of the sifted flour with each egg to prevent curdling. Stir in the vanilla extract.

Gently fold in the remaining sifted flour. If the batter seems a little stiff, stir in the tablespoon of milk.

Spoon half of the cake batter into the prepared cake tin and spread it evenly. Dollop spoonfuls of the cooled rhubarb mixture over the batter. Swirl gently with a skewer or knife.

Spoon the remaining cake batter over the rhubarb and spread it to cover. Place the remaining rhubarb mixture on top and swirl again. Try not to overmix the swirls.

Bake for 40-45 minutes, or until the cake is golden brown and a skewer inserted into the center comes out clean. Allow the cake to cool in the tin for 10 minutes before turning it out onto a wire rack to cool completely.