Line a large rimmed baking sheet with parchment paper; set aside.
Over low heat, add the butter to a saucepan. Once the butter starts to melt, add the milk and water and heat just until the milk is warm. Add the liquid to a glass bowl and stir in the sour cream. Sprinkle the top with yeast, and sugar; whisk to combine. Allow yeast to activate, about 3 minutes.
Add the flour and salt and mix with your hand or stand-up mixer until the dough is incorporated. Cover the bowl and rise for 1 1/2 to 2 hours, until the dough has doubled.
Divide the dough into four parts. Form each part into a ball-like shape.
On a lightly floured surface, gently press down into the center of the dough and pull the edges out. Keep doing this until the dough is stretched to about 7”-8” in diameter. The center of the dough should be flattened to about ¼-inch, but not the edges.
Transfer the bread to the prepared baking sheet.
Beat egg yolk and milk for the glaze.
With a pastry brush, apply the glaze to the tops and sides of the lepeshki.
Sprinkle sesame seeds in the middle. Cover the bread and allow to rise until doubled in size, about 30-45 minutes.
Bake in a preheated oven to 365°F for about 25-30 minutes. The lepeshki should be golden brown and hard to the touch. Once cooled, they will soften.