Prepare the ube base: In a jar, combine oats, milk, yogurt, ube powder, vanilla, and sweetener. Stir until the mixture becomes beautifully purple. The color transformation is immediate and quite magical, turning your simple oat mixture into something that looks like it belongs in a trendy café.
Make the chocolate center: In a small bowl, mix cocoa, milk, and syrup until you reach a thick, glossy consistency. The mixture should be pourable but substantial enough to maintain its position in the jar. Pour it gently into the center of the oatmeal mixture, don't stir too much! You want to see distinct swirls and layers, not a uniform chocolate-ube blend.
Prepare the raspberry purée: In a small saucepan, heat raspberries, water, lemon juice, and sugar over medium heat. Mash lightly with a fork or potato masher, leaving a few pieces for texture. The sauce is ready when it's slightly thickened and the raspberries have broken down but still have some body. Let cool before adding to prevent warming your cold oat mixture.
Assemble: Pour the raspberry purée over the jar as the final layer. The contrast of the bright red against the purple and chocolate is genuinely stunning. Seal your jar with a tight-fitting lid and refrigerate for at least six hours or overnight. The longer it sits, the creamier and more flavorful it becomes.