Heat oven to 350°F. Combine 3 tablespoons water, sugar, and a pinch of salt in a small saucepan over medium-high heat. Without stirring, melt sugar until completely dissolved, about 5 minutes. Once sugar is melted, turn heat to medium-low and continue to cook until it has a deep amber color, gently swirling the pan occasionally, 15 to 18 minutes. Adjust heat to low, if necessary.
As soon as the caramel reaches a deep amber color, lower the heat, add the ground cinnamon and cayenne, and swirl the pan vigorously to combine. Then immediately pour the caramel into an 8-inch cake pan, or divide evenly amongst llaneras or ramekins. Allow the caramel to cool completely.
While the caramel cools, in a large bowl, combine egg yolks, horchata concentrate, and evaporated milk. Very gently whisk in circular motions. The harder you whisk, the more bubbles will form in your custard, leaving bubbles in the finished product.
Gently pour the mixture through a mesh strainer into a measuring cup. You should have about 4 cups of mixture. Allow mixture to sit to settle any bubbles that were formed. Pour mixture into the cake pan or divide mixture evenly into llaneras or ramekins.
Place the flan pan inside a roasting pan, then place the roasting pan in the oven. Add simmering water to the roasting pan so it surrounds the flan pan with about 1 inch of water. Bake flan until firm around the edges and still wobbly in the center, 40 to 45 minutes. Bake times will be less for a llanera or ramekins, depending on their size.
Remove flan pan from water bath and let cool until room temperature. Transfer to the fridge and let set, about 4 hours. When ready to serve, remove flan from fridge and let sit 10 minutes. Run a knife around the edges and place a serving platter upside down over the top. Invert flan onto platter, scraping out any loose caramel.