First make dough. In base of food processor (or by hand), combine flour, salt, baking powder and pulse few times to whisk together. Add olive oil and butter and pulse just until combined. Add egg and water and pulse until dough resembles coarse crumbs. Turn dough out onto floured surface and knead by hand for 5 minutes until smooth. The dough should be slightly sticky, but still workable. If necessary, add small amount of flour or water to get right consistency. Form into ball and wrap in plastic wrap. Place in refrigerator to chill for 1 hour.
While dough chills, make mac and cheese. Bring large pot of salted water to boil and cook pasta according to directions until it is al dente. Drain and set aside.
While pasta cooks, melt butter over low heat in heavy bottomed saucepan. Add flour to butter and use whisk to combine quickly until it all pulls away from sides. Keep whisking while gently pouring in warm milk. The flour mixture should dissolve in milk. Continue to whisk, raising the heat to medium, until milk thickens (about 5 minutes). Add salt, pepper and nutmeg and stir until well combined.
Remove from heat. Add shredded cheese and heavy cream and keep stirring until cheese is melted. Add cooked pasta and stir to coat. Taste and adjust seasoning. Set aside.
Remove dough from fridge. Spread flour over work surface and working with about 1/4 cup size amount of dough at a time, roll into ball then use rolling pin to roll out into thin circle. Spread about teaspoon of guava paste or jam in center and top with small amount of mac and cheese. Brush edges with egg wash. Fold pastry over in half and use fork to pinch shut. Place on parchment lined baking sheet and repeat with test of dough.
To cook you can either bake or fry.
To Bake: Preheat oven to 400°F. Lightly brush top of empanadas with remaining egg wash and bake for about 20 minutes or until golden brown.
To Fry: In heavy bottom skillet or Dutch oven, heat 3/4 inch of vegetable oil to 350°F. Fry empanadas 2 or 3 at a time until golden (about 5 minutes per batch). Serve immediately.