Preheat your oven to 350°F (175°C) and line your cake pan with parchment paper or butter it well to prevent sticking.
In a large mixing bowl, cream together softened butter and sugar until the mixture becomes light, fluffy, and pale—about 3-4 minutes with a hand mixer or whisk.
Add eggs one at a time, beating well after each addition to incorporate air and create a smooth batter with a glossy appearance.
Gently mix in the milk to loosen the batter, making it easier to combine the dry ingredients evenly.
Gradually sift the flour into the wet mixture, folding gently with a spatula or mixing on low speed until just combined—avoid overmixing to keep the cake tender.
Divide the batter evenly into two bowls. To one, fold in the diced rhubarb gently; to the other, fold in the strawberry puree for a vibrant swirl effect.
Spoon dollops of the rhubarb batter into the prepared pan, then add spoonfuls of the strawberry batter on top. Use a toothpick or skewer to gently swirl the two batters together, creating marbled patterns.
Bake in the preheated oven for 45 minutes, or until a toothpick inserted into the center comes out clean and the top is golden brown.
Once baked, remove the cake from the oven and let it cool in the pan for about 15 minutes to set, then transfer it to a wire rack to cool completely.
Sprinkle a little powdered sugar over the cooled cake for a touch of sweetness and visual appeal.
Slice into pieces, revealing the striking marbled pattern of ruby and blush, and enjoy this vibrant, tart-sweet dessert.