Make the muffins first. Mix oil, eggs, vanilla, milk, sugar, cinnamon, and salt until well combined.
Add in flour and baking powder. Mix until almost completely combined. If flour is still a little streaky, that’s totally ok.
Add in blackberries and rhubarb and finish mixing. Mix until just combined.
Using a quarter cup measure, scoop batter into greased and/or lined muffin tins.
After scooping all the muffin batter, the topping can be made in the same bowl, or can be made separately to your preference.
For the topping, mix all ingredients together and scoop about 1 tbsp of topping onto each muffin. Muffins will look very full but you’ll get some really lovely muffin tops!
Bake at 350F for a total of 60 minutes. Bake covered for the first 40 minutes and uncover for the last 20 minutes. Verify done-ness using a toothpick. If it comes out clean, it's done!