Prepare the crumble topping by melting butter in a small pot over low heat. Let it cool for 5 minutes.

Mix 1/2 cup flour, 1/2 cup brown sugar, 1/2 tsp salt, and 1 tsp cinnamon powder into the melted butter using a fork until crumbly.

Prepare the wet ingredients by melting butter in the microwave for 30 seconds. Add sour cream, eggs, milk, and vanilla extract. Mix well.

In a large bowl, combine 2 cups flour, 1/2 cup brown sugar, 1 tbsp baking powder, and 1 tsp salt.

Add the wet ingredients to the dry ingredients and mix gently with a spatula until just combined.

Prepare muffin tins with paper liners. Scoop batter into each liner, add a layer of crumble topping, and repeat.

Bake in a preheated oven at 180°C (356°F) for 20-25 minutes. Check doneness with a toothpick.

Let muffins cool completely at room temperature before serving.