For the hot-dipped chicken: In a shallow baking dish or large freezer bag, combine the chicken and buttermilk. Cover or seal and let soak in the refrigerator overnight.
For the cool cucumber salad: Combine the vinegar, sugar and 1/2 cup water in a pan and bring to boil over medium-high heat. Pour over the cucumbers and onions in a heatproof container. Add the tarragon and salt, and gently toss. Refrigerate for several hours before serving.
Heat some vegetable oil in a large cast-iron skillet until the oil registers 350 degrees F on a deep-frying thermometer.
Remove the chicken from the buttermilk and dredge it in flour. Add the chicken to the hot oil and cook, flipping once, until the chicken is golden brown and a meat thermometer registers 165 degrees F, about 20 minutes. Drain the chicken on paper towels.
Mix the maple syrup, sambal and hot sauce in a bowl. Dip the chicken into the hot sauce mixture to coat well.
Top each garlic Texas toast with some cucumber salad and a hot-dipped chicken breast.