Preheat oven and arrange bread:

Heat oven to 350°F (175°C). Lay bread slices on a broiler-safe baking sheet that is ungreased and unlined.

Mix cinnamon butter paste:

In a medium bowl, mix together softened butter, sugar, cinnamon, and vanilla using a fork until a smooth paste forms.

Spread butter on bread:

Evenly distribute the cinnamon butter across all bread slices, spreading it edge-to-edge to fully cover each slice.

Bake bread until crisp:

Position the baking sheet on the top or center rack (upper third of oven, at least 5 inches below broiler to avoid scorching). Bake for 10 minutes until the bread turns lightly crisp.

Broil until bubbling:

Switch broiler to high and broil until the cinnamon sugar bubbles, about 1 to 2 minutes. Keep a close eye; if edges start to char, take out at once.

Cool and serve:

Allow toast to cool a bit, then cut into pieces and serve.