To make the dough: Place milk and 4 tablespoons of butter in a small saucepan set over medium-low heat. Continue to heat until butter has melted and milk begins to steam. Remove from heat and pour into a mixing bowl of a stand mixer or a large mixing bowl. Add in 180 grams of the flour, sugar, yeast, and salt and stir until no dry flour remains. Cover bowl and let sit until bubbles form and dough looks porous, about 30 minutes.

In a small bowl, whisk together oil and egg yolks. Pour egg mixture into bowl with dough and stir to combine. Add in remaining 240 grams of flour and, using a dough hook, knead dough on medium speed until dough is smooth and elastic, about 5 minutes. Alternatively, knead dough by hand on a lightly floured cutting board until smooth and elastic. If using a stand mixer, remove dough from bowl and knead by hand on a lightly floured cutting board into a smooth ball.

Place dough ball in a lightly oiled bowl, cover with plastic wrap, and let rise until doubled in volume, between 1 and 3 hours.

To make the kolaches: Turn dough out onto a lightly floured cutting board and divide into 12 equal pieces. Take one piece of dough and stretch it out into a roughly 4-inch round. In the middle of dough round, place roughly 1 ounce of chopped brisket, ½ an ounce of grated cheese, and 1 teaspoon of chopped jalapeños. Bring sides of dough up around filling and pinch close to seal. Place filled dough on a parchment-lined baking sheet, seam side down, and press a single jalapeño slice into top of dough (if using). Repeat with remaining dough and filling ingredients. Cover filled kolaches with a kitchen towel and let rise for 45 minutes.

While the filled dough is rising, preheat oven to 375°F. Metl remaining 4 tablespoons of butter. Remove towel and brush tops and sides of kolaches with about ½ of the melted butter. Transfer baking sheet to oven and cook until lightly browned, about 15 to 18 minutes. Remove baking sheet from oven and brush kolaches again with remaining melted butter. Let kolaches cool for 5 minutes, then serve while warm. Store leftover kolaches in refrigerator for up to 3 days, reheating as needed.