Preheat oven to 350 degrees F with rack in middle. Butter an 8-inch square cake pan and line with a square of parchment paper, then butter parchment. Sift together flour, baking powder, baking soda, and salt. Beat cannabis butter and sugar in a large bowl with an electric mixer at medium speed until pale and fluffy, then beat in vanilla. Add eggs 1 at a time, beating well after each addition. At low speed, beat in buttermilk until just combined (mixture may look curdled). Add flour mixture in 3 batches, mixing until each addition is just incorporated. Spread batter evenly in cake pan, then rap pan on counter several times to eliminate air bubbles. Bake until golden and a wooden pick inserted in center of cake comes out clean, 35 to 40 minutes. Cool in pan on a rack 10 minutes, then run a knife around edge of pan. Invert onto rack and discard parchment, then cool completely, about 1 hour.
Make glaze: Bring cream, brown sugar, corn syrup, and a pinch of salt to a boil in a medium heavy saucepan over medium heat, stirring until sugar has dissolved. Boil until glaze registers 210 to 212 degrees F on a thermometer, 12 to 14 minutes, then stir in vanilla. Put rack with cake in a shallow baking pan and pour hot glaze over top of cake, allowing it to run down sides. Cool until glaze is set, about 30 minutes.