I know it’s a very hot summer in places right now but these muffins are completely worth it.
(Sorry for mixing unit types here, but these are from my notes and what I know works for me)

375g AP flour
265g Brown sugar (I prefer dark)
2.5tsp Baking powder (from a can opened <6mo ago)
0.5tsp Baking soda
0.25tsp cinnamon (ground fresh ceylon)
0.25tsp nutmeg (ground fresh)
0.25tsp salt
1.0 egg
1.25cup buttermilk
0.5cup unsalted butter, melted and cooled
2.0tsp vanilla extract
1.0cup raspberries
1.0cup rhubarb, choppped

Preheat oven to 400F

0.5cup chopped pecans
0.33cup brown sugar, packed
0.25tsp cinnamon
1.0Tbsp cold butter

Combine ingredients in food processor, pulse until they resemble coarse crumbs

Whisk together flour, baking {powder,soda}, cinnamon, nutmeg, and salt
In a separate bowl combine sugar, buttermilk, butter, egg, and vanilla
Combine the two mixtures, mixing until roughly combined
Add in fruit, stir three times to distribute
Pour into muffin liners in a muffin tin (makes 12-18)
Sprinkle topping onto batter
Bake @400F for 20-25min

Notes on muffins:
Fresh ground spices are THE BEST. Get a microplane for ~$10 and you’ll be set.
Nothing in the world smells as good as fresh ground cinnamon and nutmeg.

Having fresh baking powder is important. It reacts with air and loses potentcy.
Muffins are chemically levened by the reaction between the acidic buttermilk
and tartaric acid in baking powder and the basic baking soda.

Muffins get their texture from being roughly mixed. You only need to mix until
all the flour is wet; any more and you risk developing gluten strands and
making your muffins chewy, with a cake-like crumb.