In bowl of stand mixer with hook attachment mix flour, butter, egg yolk, sugar, salt and water.
Knead dough on speed 2 until it separates from bowl. Remove dough from bowl and manually knead on a lightly floured surface until smooth, about 2 minutes.
Cut dough in half and cover. Roll out one half of dough until 1/8 inch thin on a lightly floured surface.
Cut dough into 3-inch squares.
Cut jalapeños in half, then in half again, until you have four similarly sized slices. Cut the slices into 1-inch portions. Pieces that are too small will be minced.
Using one square of dough at a time, place a 1-inch slice of jalapeño in center. Top with a cube of queso blanco and finish off with a sprinkling of minced jalapeño.
Lift right and left sides of square up and pinch together to seal. Repeat with top and bottom sides. Pinch all openings closed to seal.
Repeat this process with remaining dough.
Heat 1-inch of oil in a frying pan over medium-high heat. Fry poppers in batches, flipping after 30 seconds, until crispy and golden on both sides.
With a slotted spoon remove poppers and drain on paper towels.