Preheat oven to 350 degrees, and prepare a 12 muffin tin.
First make the streusel topping. Partially melt the butter, or use it extremely soft, then simply mix all ingredients together. Scrape out into a small bowl, and set aside.
Continuing in the same mixer bowl, beat the butter and sugar until creamy. Add in the eggs, scraping the sides if necessary, then the kefir and vanilla.
Add in all the dry ingredients, slowly increasing the speed to beat the batter for several minutes until pale yellow, and slightly fluffy.
Place about 3-4 tablespoons of batter into the bottom of each muffin space. I use my favorite muffin scoop for ease and consistency. Scoop about a teaspoon of the streusel mix into the center (mixture should be soft so it is a bit easier to swirl). Using a butter knife, gently swirl and stab to disperse the filling a bit.
Now add another dollop of batter on top of the streusel. Divide the reaming filling over the tops of the all the muffins, gently swirling again if desired.
Bake in a preheated oven for 20-22 minutes until the muffins are get and golden.
Let rest a few minutes before enjoying. These store well at room temperature for multiple days...IF they last that long!