Put a dry frying pan over a high heat and add the pancetta. Fry until the pancetta is crisp and golden then leave to cool slightly. Tip the flour, baking powder, sugar and salt into a large mixing bowl. Crack in the eggs and whisk while slowly pouring in the milk and the melted butter, keeping the batter smooth. Stir in the pancetta and leave the batter to rest while you make the cheese sauce.
Pour the 375ml milk into a medium saucepan with the peeled, halved onion and the bay leaf, then bring to a simmer over a medium heat and strain out the flavourings.
Put another medium saucepan over a medium heat and add the 3 tbsp butter. Let it melt and bubble then add the flour. Cook for a minute so that it thickens to a roux. Pour in the warmed milk a little at a time, stirring until smooth each time before adding more. Take the saucepan off the heat, grate all of the cheeses and add them to the sauce. Whisk them in and season with salt and pepper to taste.
Quarter the tomatoes and cut out the pulp then chop the flesh into small dice. De-seed and chop the pepper into small dice. Peel and dice the onion as fine as possible and finely chop the jalapeños, avoiding the seeds. Mix it all together in a bowl with a squeeze of lime and pinches of salt and pepper. Leave on the side for later.
Put a small saucepan over a low heat and add the butter, chilli powder and paprika. Peel and crush the garlic clove (keeping it whole) and add it to the pan. Melt the butter and let it bubble gently for a minute then leave for 10 minutes to allow the garlic to infuse. Scoop out the garlic clove and set aside.
Heat an ovenproof non-stick frying pan over a medium-high heat and spread a little extra butter around the pan. Pour the batter into roughly 20cm circles. Cook for 3-4 minutes, until the edges of the pancake begin to cook then flip and cook for the same time on the other side. Remove the pancakes from the pan and repeat until you have used all of the batter.
Use a metal ring cutter to cut out the middles from the pancake, leaving 2 whole. Stack them up on your serving plate, with a whole one on the bottom, aligning the holes in the middles. Carefully pour the cheese sauce into the hole then cover with the second whole pancake. Top with the salsa and the smoky chilli butter and serve.