In a medium bowl, mix the cayenne, paprika, smoked paprika, garlic powder, onion powder and salt. Add the cornstarch, mix well, and set aside.
Toss the cauliflower in the spiced cornstarch, lightly coating each piece. In another medium bowl, whisk together the buttermilk and hot sauce. Shaking off most of the cornstarch from the cauliflower, transfer them to the buttermilk mixture, tossing them to coat. Dip one cauliflower steak at a time back into the spiced flour and then place on a baking sheet or large plate. Repeat with the remaining pieces and then let sit for 10 minutes, allowing the buttermilk to hydrate the cornstarch for a crispier crust
While the cauliflower is resting, make the aioli by whisking together the mayo, lemon zest, lemon juice and salt. Refrigerate until ready to serve.
To fry the cauliflower, add a couple inches of canola oil to a large skillet and heat over medium-high heat to about 350°F. Fry the cauliflower in batches until golden-brown and crisp, about 4 to 5 minutes, flipping halfway through.
To assemble the sandwiches, place the cauliflower steak on the bottom bun. Add a few pickle chips. Generously spread the aioli on the top bun and place on the sandwich. Repeat with the remaining sandwiches and serve immediately.