In a medium bowl sift flour, salt, and baking powder. Using a pastry blender, cut butter into flour until well incorporated. Once mixture is in crumbles, drizzle in ice water while continuing to mix with hands.
Knead dough until soft and pliable. Wrap in plastic wrap and refrigerate for at least 1 hour.
Preheat the oven to 400F. Melt the serrano butter in a sauté pan set over medium-high heat. Add the leeks and cook, stirring until leeks have softened. Remove from heat and cool.
Roll out the dough on a floured surface to 1/8-inch-thick. Cut out 8 6-inch rounds, gathering and rerolling scraps as needed.
Working with one piece of dough at a time, place 1 tablespoon of leeks into the center of a slice of ham. Roll up ham, as you would a burrito, and place in the center of the dough round.
Bring the top of the round down to meet the bottom edge, making a half moon shape. Press the edges down using your fingertips.
Line a baking sheet with a silicone baking mat and place empanadas on the sheet. Whisk egg in a small bowl and brush tops and edges of empanadas with egg wash. Repeat with remaining rounds of dough and filling.
Bake empanadas until golden brown on top, about 12 minutes. Serve warm.