Whisk together the egg yolks and cornstarch until they're a smooth mixture. Set aside.
Stir the kefir and granulated sugar in a medium saucepan over low heat until it's almost hot. Gradually pour about ¼ cup of the warm mixture into the egg/cornstarch mixture while stirring constantly. Then, transfer the egg/cornstarch mixture into the pan of warm kefir while stirring constantly. Heat on low-medium while stirring until it's thick and smooth.
Remove from heat and stir in the vanilla extract and butter until well combined. Cover the top of the pudding with plastic wrap (so that it's touching the pudding) and let it cool.
Blueberry Pie Filling
Mix the lemon juice and cornstarch together. Add a bit of water if needed to help dissolve the cornstarch better.
Heat the blueberries and light brown sugar in a medium saucepan over low-medium heat until the berries are soft and bursting. Mix in the lemon juice/cornstarch and continue to heat until it's thick. Let it cool.
Assembling the Jars
When the pudding and pie filling have cooled to room temperature and the jars are ready to be served, assemble the jars. (See note below to make ahead & assemble later).
Alternately layer pie crust crumbs, pudding, and pie filling until the jars are full. Enjoy!
Notes
Store leftover jars in the refrigerator for up to 2 days. For best results, consume the day of.To make ahead, prepare the pudding and pie filling and refrigerate them separately for up to 1 week. Assemble the jars the day you'll be serving them.The provided nutrition information is an estimate, the calorie accuracy is not guaranteed.