Combine the half-and-half, cream cheese, sour cream, condensed milk, vanilla extract and salt in a food processor and process until smooth. Refrigerate until ready to use.
For the guava syrup:
Combine the guava paste, cream soda and salt in a saucepan and heat gently, until the guava paste has melted into a syrup. (For an extra-smooth consistency, use a hand blender to puree any unmelted guava solids.) You will need 2 ounces of the guava syrup for the torrejas. Leftover syrup will keep in the refrigerator for at least 3 weeks.
For the cookie sugar:
Pulse the Maria cookies in a food processor until crumbled. Add the brown sugar and salt and pulse to combine.
For the torrejas:
Whisk together the half-and-half, eggs, sugar, vanilla extract and salt in a medium mixing bowl (this can be done the night before), and pour the custard mixture into a pie pan.
Soak the bread in the egg mixture for 30 seconds on each side, then remove to a rack set over a baking sheet to drain for a minute or two.
Over medium-low heat, melt 2 tablespoons of the butter in a medium nonstick skillet. Fry the torrejas, two slices at a time, until golden brown, 2 to 3 minutes per side. Melt the remaining 2 tablespoons butter in the skillet before starting the second batch of torrejas.
To serve, place two torrejas on a plate. Drizzle generously with cream cheese sauce, then with guava syrup. Top with a sprinkling of Maria cookie sugar and some powdered sugar.