Preheat oven to 350 F. Line a round 8″ baking pan with parchment paper.
In a bowl, whisk together flour, sugar & baking soda. With fingers, cut in cold butter & cream cheese until mixture resembles small peas. Do NOT over work dough. Carefully stir in guava paste cubes with a fork.
In a small cup, beat egg slightly then combine with buttermilk & vanilla. Add wet ingredients to flour mixture, stirring ONLY until combined.
Pour dough into lined baking pan, spreading evenly. Sprinkle with sanding sugar. Bake about 20 minutes or until golden & tests done. Slice into 8 wedges & serve warm.