01 - Set oven to 375°F and line muffin tin with paper liners.
02 - Combine flour, baking powder, baking soda, salt, cinnamon, ginger, cardamom, cloves, and nutmeg in a medium bowl.
03 - Whisk granulated sugar, brown sugar, vegetable oil, eggs, and vanilla extract in a large bowl until homogenous.
04 - Add half the dry mixture to the wet ingredients, stirring gently. Incorporate the milk, then fold in remaining dry ingredients until just mixed.
05 - Carefully fold in chopped plums, dispersing evenly throughout the batter.
06 - Portion batter evenly among the lined muffin cups, filling each about three-quarters full.
07 - Mix flour, brown sugar, spices, and salt in a separate bowl. Cut in cold butter using a fork or fingertips until coarse crumbs form.
08 - Generously distribute the chai crumble over each batter-filled muffin cup.
09 - Bake in preheated oven for 22–25 minutes, or until a toothpick inserted in the center emerges clean.
10 - Allow muffins to rest in the tin for 5 minutes, then transfer to a wire rack for complete cooling.